From 3904f44ed928df08afc076872904caa53eeed450 Mon Sep 17 00:00:00 2001
From: Joshua MacDonald <jmacd@lightstep.com>
Date: Fri, 23 Jul 2021 11:00:21 -0700
Subject: [PATCH 01/13] Draft base-2 exponential histogram proto

---
 CHANGELOG.md                                 |   2 +-
 opentelemetry/proto/metrics/v1/metrics.proto | 138 +++++++++++++++++++
 2 files changed, 139 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index bf897bdb6..f0623ee36 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,7 +16,7 @@ Full list of differences found in [this compare.](https://github.com/open-teleme
 
 ### Added
 
-* Remove if no changes for this section before release.
+* ExponentialHistogram is a base-2 exponential histogram described in [OTEP 149](https://github.com/open-telemetry/oteps/pull/149).
 
 ### Removed
 
diff --git a/opentelemetry/proto/metrics/v1/metrics.proto b/opentelemetry/proto/metrics/v1/metrics.proto
index 0db113e63..67297d439 100644
--- a/opentelemetry/proto/metrics/v1/metrics.proto
+++ b/opentelemetry/proto/metrics/v1/metrics.proto
@@ -221,6 +221,16 @@ message Histogram {
   AggregationTemporality aggregation_temporality = 2;
 }
 
+// ExponentialHistogram represents the type of a metric that is calculated by aggregating
+// as a ExponentialHistogram of all reported double measurements over a time interval.
+message ExponentialHistogram {
+  repeated ExponentialHistogramDataPoint data_points = 1;
+
+  // aggregation_temporality describes if the aggregator reports delta changes
+  // since last report time, or cumulative changes since a fixed start time.
+  AggregationTemporality aggregation_temporality = 2;
+}
+
 // Summary metric data are used to convey quantile summaries,
 // a Prometheus (see: https://prometheus.io/docs/concepts/metric_types/#summary)
 // and OpenMetrics (see: https://github.com/OpenObservability/OpenMetrics/blob/4dbf6075567ab43296eed941037c12951faafb92/protos/prometheus.proto#L45)
@@ -431,6 +441,134 @@ message HistogramDataPoint {
   repeated Exemplar exemplars = 8;
 }
 
+// ExponentialHistogramDataPoint is a single data point in a timeseries that describes the
+// time-varying values of a ExponentialHistogram of double values. A ExponentialHistogram contains
+// summary statistics for a population of values, it may optionally contain the
+// distribution of those values across a set of buckets.
+//
+message ExponentialHistogramDataPoint {
+  // The set of key/value pairs that uniquely identify the timeseries from
+  // where this point belongs. The list may be empty (may contain 0 elements).
+  repeated opentelemetry.proto.common.v1.KeyValue attributes = 1;
+
+  // StartTimeUnixNano is optional but strongly encouraged, see the
+  // the detiled comments above Metric.
+  //
+  // Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January
+  // 1970.
+  fixed64 start_time_unix_nano = 2;
+
+  // TimeUnixNano is required, see the detailed comments above Metric.
+  //
+  // Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January
+  // 1970.
+  fixed64 time_unix_nano = 3;
+
+  // count is the number of values in the population. Must be non-negative. This
+  // value must be equal to the sum of the "count" fields in buckets if a
+  // histogram is provided.
+  fixed64 count = 4;
+
+  // sum of the values in the population. If count is zero then this field
+  // must be zero. This value must be equal to the sum of the "sum" fields in
+  // buckets if a histogram is provided.
+  //
+  // Note: Sum should only be filled out when measuring non-negative discrete
+  // events, and is assumed to be monotonic over the values of these events.
+  // Negative events *can* be recorded, but sum should not be filled out when
+  // doing so.  This is specifically to enforce compatibility w/ OpenMetrics,
+  // see: https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#histogram
+  double sum = 5;
+
+  // scale describes the resolution of the histogram.  Boundaries are
+  // located at powers of the base, where:
+  //
+  //   base = (2^(2^-scale))
+  //
+  // The histogram bucket identified by `index`, a signed integer,
+  // contains values that are less than or equal to (base^index) and
+  // greater than (base^(index-1)).
+  //
+  // The positive and negative ranges of the histogram are expressed
+  // separately but both use the same scale to locate their boundaries.
+  //
+  // scale is valid in the range -4 through +8 inclusive.
+  sint32 scale = 6;
+
+  // zero_count is the count of values that are either exactly zero or
+  // within the region considered zero by the instrumentation at the
+  // tolerated degree of precision.  This bucket stores values that
+  // cannot be expressed using the standard exponential formula as
+  // well as values that have been rounded to zero.  Users have the
+  // option to set zero_tolerance to convey additional information about
+  // the width of the zero region.
+  //
+  // Implementations MAY consider the zero bucket to have probability
+  // mass equal to (zero_count / sum).
+  fixed64 zero_count = 7;
+
+  // zero_tolerance may be optionally set to convey the width of the
+  // zero region.  When this is not set, implementations may assume
+  // the zero tolerance equals the smallest representable positive
+  // IEEE 754 double-precision floating point number, which is (2^−1074).
+  //
+  // Producers can be configured to round measurements to zero that fall
+  // below the zero tolerance.  Users should avoid producing measurements
+  // that have meaningless precision near zero, as exponential buckets
+  // become increasingly dense in this region.
+  //
+  // Implementations MAY consider the zero bucket to have probability
+  // density equal to (zero_count / 2*zero_tolerance), if set.
+  double zero_tolerance = 8;
+
+  // positive carries the positive range of exponential bucket counts.
+  SparseBuckets positive = 9;
+
+  // negative carries the negative range of exponential bucket counts.
+  SparseBuckets negative = 10;
+
+  // SparseBuckets are a sparse set of bucket counts, compactly encoded.
+  message SparseBuckets {
+    // Span encodes a run of contiguous histogram buckets.
+    message Span {
+      sint32 offset = 1; // Gap to previous span, or starting point for 1st span (which can be negative).
+      uint32 length = 2; // Length of consecutive buckets.
+    }
+
+    // span is a repeated list of Spans that encodes a range of
+    // exponential buckets, positive or negative.  The offset of the
+    // first Span indicates the smallest index in the Histogram with
+    // non-zero count.  Subsequent contiguous buckets will be included
+    // in the same Span, and subsequent Spans encode additional
+    // buckets separated by gaps where the histogram has zero count.
+    //
+    // The starting index of a Span equals its own offset plus the
+    // starting index of the previous Span plus the previous Span's
+    // length.  As a recursive formula:
+    //
+    //    starting_index(span[x+1]) =
+    //       starting_index(span[x]) +
+    //       span[x+1].offset +
+    //       span[x].length
+    //
+    // For example, the two Spans [ {offset: 10, length: 3}, {offset:
+    // 5, length: 8} ] indicate a total of 11 buckets with indices at
+    // [10, 11, 12] and [18, 19, 20, 21, 22, 23, 24, 25].
+    repeated Span span = 1;
+
+    // delta is an array of counts corresponding to the buckets
+    // described by `span`, encoded as deltas.  The indicated count 
+    // for bucket[x] is defined as a recursive formula:
+    //
+    //    bucket_count(x+1) = bucket_count(x) + delta[x+1]
+    repeated sint64 delta = 2;
+  } 
+
+  // (Optional) List of exemplars collected from
+  // measurements that were used to form the data point
+  repeated Exemplar exemplars = 8;
+}
+
 // SummaryDataPoint is a single data point in a timeseries that describes the
 // time-varying values of a Summary metric.
 message SummaryDataPoint {

From e680b5938be129ac8410f7ca66f0a50bcc4c4b52 Mon Sep 17 00:00:00 2001
From: Joshua MacDonald <jmacd@lightstep.com>
Date: Mon, 26 Jul 2021 16:44:03 -0700
Subject: [PATCH 02/13] update exemplars number

---
 opentelemetry/proto/metrics/v1/metrics.proto | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/opentelemetry/proto/metrics/v1/metrics.proto b/opentelemetry/proto/metrics/v1/metrics.proto
index 67297d439..49b448a52 100644
--- a/opentelemetry/proto/metrics/v1/metrics.proto
+++ b/opentelemetry/proto/metrics/v1/metrics.proto
@@ -566,7 +566,7 @@ message ExponentialHistogramDataPoint {
 
   // (Optional) List of exemplars collected from
   // measurements that were used to form the data point
-  repeated Exemplar exemplars = 8;
+  repeated Exemplar exemplars = 11;
 }
 
 // SummaryDataPoint is a single data point in a timeseries that describes the

From b368c474f050a29352fa0e150ac64860bd2e78ef Mon Sep 17 00:00:00 2001
From: Joshua MacDonald <jmacd@lightstep.com>
Date: Mon, 9 Aug 2021 11:45:57 -0700
Subject: [PATCH 03/13] two clarifications; unrestrict scale

---
 opentelemetry/proto/metrics/v1/metrics.proto | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/opentelemetry/proto/metrics/v1/metrics.proto b/opentelemetry/proto/metrics/v1/metrics.proto
index 49b448a52..29a375e8c 100644
--- a/opentelemetry/proto/metrics/v1/metrics.proto
+++ b/opentelemetry/proto/metrics/v1/metrics.proto
@@ -490,9 +490,11 @@ message ExponentialHistogramDataPoint {
   // greater than (base^(index-1)).
   //
   // The positive and negative ranges of the histogram are expressed
-  // separately but both use the same scale to locate their boundaries.
+  // separately.  Negative values are mapped by their absolute value
+  // into the negative range using the same scale as the positive range.
   //
-  // scale is valid in the range -4 through +8 inclusive.
+  // scale is not restricted by the protocol, as the permissible
+  // values depend on the range of the data.
   sint32 scale = 6;
 
   // zero_count is the count of values that are either exactly zero or
@@ -504,7 +506,7 @@ message ExponentialHistogramDataPoint {
   // the width of the zero region.
   //
   // Implementations MAY consider the zero bucket to have probability
-  // mass equal to (zero_count / sum).
+  // mass equal to (zero_count / count).
   fixed64 zero_count = 7;
 
   // zero_tolerance may be optionally set to convey the width of the

From e62badd7a8ddee0bb2e656d6548a5e000baf9eef Mon Sep 17 00:00:00 2001
From: Joshua MacDonald <jmacd@lightstep.com>
Date: Mon, 9 Aug 2021 13:53:37 -0700
Subject: [PATCH 04/13] from feedback; remove sparse encoding; remove zero
 tolerance; keep exclusive lower bound

---
 opentelemetry/proto/metrics/v1/metrics.proto | 111 +++++--------------
 1 file changed, 26 insertions(+), 85 deletions(-)

diff --git a/opentelemetry/proto/metrics/v1/metrics.proto b/opentelemetry/proto/metrics/v1/metrics.proto
index 29a375e8c..166695365 100644
--- a/opentelemetry/proto/metrics/v1/metrics.proto
+++ b/opentelemetry/proto/metrics/v1/metrics.proto
@@ -447,39 +447,11 @@ message HistogramDataPoint {
 // distribution of those values across a set of buckets.
 //
 message ExponentialHistogramDataPoint {
-  // The set of key/value pairs that uniquely identify the timeseries from
-  // where this point belongs. The list may be empty (may contain 0 elements).
-  repeated opentelemetry.proto.common.v1.KeyValue attributes = 1;
-
-  // StartTimeUnixNano is optional but strongly encouraged, see the
-  // the detiled comments above Metric.
-  //
-  // Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January
-  // 1970.
-  fixed64 start_time_unix_nano = 2;
-
-  // TimeUnixNano is required, see the detailed comments above Metric.
-  //
-  // Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January
-  // 1970.
-  fixed64 time_unix_nano = 3;
-
-  // count is the number of values in the population. Must be non-negative. This
-  // value must be equal to the sum of the "count" fields in buckets if a
-  // histogram is provided.
-  fixed64 count = 4;
-
-  // sum of the values in the population. If count is zero then this field
-  // must be zero. This value must be equal to the sum of the "sum" fields in
-  // buckets if a histogram is provided.
-  //
-  // Note: Sum should only be filled out when measuring non-negative discrete
-  // events, and is assumed to be monotonic over the values of these events.
-  // Negative events *can* be recorded, but sum should not be filled out when
-  // doing so.  This is specifically to enforce compatibility w/ OpenMetrics,
-  // see: https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#histogram
-  double sum = 5;
-
+  // T.B.D. The Attributes, Timestamps, Flags, Count, and Sum fields will
+  // be copied from HistohgramDataPoint before merging.  These fields arte
+  // identical between these two types, and both have pending changes in the
+  // opentelemetry-proto repository.
+  
   // scale describes the resolution of the histogram.  Boundaries are
   // located at powers of the base, where:
   //
@@ -509,66 +481,35 @@ message ExponentialHistogramDataPoint {
   // mass equal to (zero_count / count).
   fixed64 zero_count = 7;
 
-  // zero_tolerance may be optionally set to convey the width of the
-  // zero region.  When this is not set, implementations may assume
-  // the zero tolerance equals the smallest representable positive
-  // IEEE 754 double-precision floating point number, which is (2^−1074).
-  //
-  // Producers can be configured to round measurements to zero that fall
-  // below the zero tolerance.  Users should avoid producing measurements
-  // that have meaningless precision near zero, as exponential buckets
-  // become increasingly dense in this region.
-  //
-  // Implementations MAY consider the zero bucket to have probability
-  // density equal to (zero_count / 2*zero_tolerance), if set.
-  double zero_tolerance = 8;
-
   // positive carries the positive range of exponential bucket counts.
-  SparseBuckets positive = 9;
+  Span positive = 8;
 
   // negative carries the negative range of exponential bucket counts.
-  SparseBuckets negative = 10;
-
-  // SparseBuckets are a sparse set of bucket counts, compactly encoded.
-  message SparseBuckets {
-    // Span encodes a run of contiguous histogram buckets.
-    message Span {
-      sint32 offset = 1; // Gap to previous span, or starting point for 1st span (which can be negative).
-      uint32 length = 2; // Length of consecutive buckets.
-    }
-
-    // span is a repeated list of Spans that encodes a range of
-    // exponential buckets, positive or negative.  The offset of the
-    // first Span indicates the smallest index in the Histogram with
-    // non-zero count.  Subsequent contiguous buckets will be included
-    // in the same Span, and subsequent Spans encode additional
-    // buckets separated by gaps where the histogram has zero count.
-    //
-    // The starting index of a Span equals its own offset plus the
-    // starting index of the previous Span plus the previous Span's
-    // length.  As a recursive formula:
-    //
-    //    starting_index(span[x+1]) =
-    //       starting_index(span[x]) +
-    //       span[x+1].offset +
-    //       span[x].length
-    //
-    // For example, the two Spans [ {offset: 10, length: 3}, {offset:
-    // 5, length: 8} ] indicate a total of 11 buckets with indices at
-    // [10, 11, 12] and [18, 19, 20, 21, 22, 23, 24, 25].
-    repeated Span span = 1;
-
-    // delta is an array of counts corresponding to the buckets
-    // described by `span`, encoded as deltas.  The indicated count 
-    // for bucket[x] is defined as a recursive formula:
+  Span negative = 9;
+
+  // Buckets are a set of bucket counts, encoded in a contiguous array
+  // of counts.
+  message Span {
+    // Offset is the index of the first count expressed in this span.
+    // 
+    // Note: This uses a varint encoding as a simple form of compression.
+    sint64 offset = 1;
+
+    // Count is an array of counts, where count[i] carries the count
+    // of the bucket at index (offset+i), i.e., count[i] is the count
+    // of values less than or equal to base^(offset+i) and greater
+    // than base^(offset+i-1).
     //
-    //    bucket_count(x+1) = bucket_count(x) + delta[x+1]
-    repeated sint64 delta = 2;
+    // Note: By contrast, the explicit HistogramDataPoint uses
+    // fixedt64.  This field is expected to have many buckets,
+    // especially zeros, so has been selected to ensure varint
+    // encoding.
+    repeated uint64 bucket_counts = 6;
   } 
 
   // (Optional) List of exemplars collected from
   // measurements that were used to form the data point
-  repeated Exemplar exemplars = 11;
+  repeated Exemplar exemplars = 10;
 }
 
 // SummaryDataPoint is a single data point in a timeseries that describes the

From 1eca83feee6f6a8d4d40944a67351535a54186eb Mon Sep 17 00:00:00 2001
From: Joshua MacDonald <jmacd@lightstep.com>
Date: Mon, 9 Aug 2021 13:57:20 -0700
Subject: [PATCH 05/13] add flags

---
 opentelemetry/proto/metrics/v1/metrics.proto | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/opentelemetry/proto/metrics/v1/metrics.proto b/opentelemetry/proto/metrics/v1/metrics.proto
index 55bd36e3b..c9157a07b 100644
--- a/opentelemetry/proto/metrics/v1/metrics.proto
+++ b/opentelemetry/proto/metrics/v1/metrics.proto
@@ -527,6 +527,10 @@ message ExponentialHistogramDataPoint {
   // (Optional) List of exemplars collected from
   // measurements that were used to form the data point
   repeated Exemplar exemplars = 10;
+
+  // Flags that apply to this specific data point.  See DataPointFlags
+  // for the available flags and their meaning.
+  uint32 flags = 11;
 }
 
 // SummaryDataPoint is a single data point in a timeseries that describes the

From e4f3b852aa65c4dde3310c40283f5d34d09eb3a0 Mon Sep 17 00:00:00 2001
From: Joshua MacDonald <jmacd@lightstep.com>
Date: Mon, 9 Aug 2021 13:59:36 -0700
Subject: [PATCH 06/13] reformat

---
 opentelemetry/proto/metrics/v1/metrics.proto | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/opentelemetry/proto/metrics/v1/metrics.proto b/opentelemetry/proto/metrics/v1/metrics.proto
index c9157a07b..14344b847 100644
--- a/opentelemetry/proto/metrics/v1/metrics.proto
+++ b/opentelemetry/proto/metrics/v1/metrics.proto
@@ -464,10 +464,10 @@ message HistogramDataPoint {
 // distribution of those values across a set of buckets.
 //
 message ExponentialHistogramDataPoint {
-  // T.B.D. The Attributes, Timestamps, Flags, Count, and Sum fields will
-  // be copied from HistohgramDataPoint before merging.  These fields arte
-  // identical between these two types, and both have pending changes in the
-  // opentelemetry-proto repository.
+  // T.B.D. The Attributes, Timestamps, Count, and Sum fields will be
+  // copied with their comments from HistohgramDataPoint before
+  // merging.  These fields arte identical between these two types,
+  // and have a pending change in the opentelemetry-proto repository.
   
   // scale describes the resolution of the histogram.  Boundaries are
   // located at powers of the base, where:

From ef3d927622b9b4beb7eb1270785e2c59836178b4 Mon Sep 17 00:00:00 2001
From: Joshua MacDonald <jmacd@lightstep.com>
Date: Mon, 9 Aug 2021 14:01:11 -0700
Subject: [PATCH 07/13] renumber

---
 opentelemetry/proto/metrics/v1/metrics.proto | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/opentelemetry/proto/metrics/v1/metrics.proto b/opentelemetry/proto/metrics/v1/metrics.proto
index 14344b847..0bcf34dce 100644
--- a/opentelemetry/proto/metrics/v1/metrics.proto
+++ b/opentelemetry/proto/metrics/v1/metrics.proto
@@ -521,7 +521,7 @@ message ExponentialHistogramDataPoint {
     // fixedt64.  This field is expected to have many buckets,
     // especially zeros, so has been selected to ensure varint
     // encoding.
-    repeated uint64 bucket_counts = 6;
+    repeated uint64 bucket_counts = 2;
   } 
 
   // (Optional) List of exemplars collected from

From 2535f0979e63ea5f168ca1d376c0d76caca78190 Mon Sep 17 00:00:00 2001
From: Joshua MacDonald <jmacd@lightstep.com>
Date: Mon, 16 Aug 2021 16:41:27 -0700
Subject: [PATCH 08/13] Rename histogram span to Buckets

---
 opentelemetry/proto/metrics/v1/metrics.proto | 42 ++++++++++++++++----
 1 file changed, 35 insertions(+), 7 deletions(-)

diff --git a/opentelemetry/proto/metrics/v1/metrics.proto b/opentelemetry/proto/metrics/v1/metrics.proto
index 0bcf34dce..1c6b269ed 100644
--- a/opentelemetry/proto/metrics/v1/metrics.proto
+++ b/opentelemetry/proto/metrics/v1/metrics.proto
@@ -464,10 +464,38 @@ message HistogramDataPoint {
 // distribution of those values across a set of buckets.
 //
 message ExponentialHistogramDataPoint {
-  // T.B.D. The Attributes, Timestamps, Count, and Sum fields will be
-  // copied with their comments from HistohgramDataPoint before
-  // merging.  These fields arte identical between these two types,
-  // and have a pending change in the opentelemetry-proto repository.
+  // The set of key/value pairs that uniquely identify the timeseries from
+  // where this point belongs. The list may be empty (may contain 0 elements).
+  repeated opentelemetry.proto.common.v1.KeyValue attributes = 1;
+
+  // StartTimeUnixNano is optional but strongly encouraged, see the
+  // the detiled comments above Metric.
+  //
+  // Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January
+  // 1970.
+  fixed64 start_time_unix_nano = 2;
+
+  // TimeUnixNano is required, see the detailed comments above Metric.
+  //
+  // Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January
+  // 1970.
+  fixed64 time_unix_nano = 3;
+
+  // count is the number of values in the population. Must be non-negative. This
+  // value must be equal to the sum of the "count" fields in buckets if a
+  // histogram is provided.
+  fixed64 count = 4;
+
+  // sum of the values in the population. If count is zero then this field
+  // must be zero. This value must be equal to the sum of the "sum" fields in
+  // buckets if a histogram is provided.
+  //
+  // Note: Sum should only be filled out when measuring non-negative discrete
+  // events, and is assumed to be monotonic over the values of these events.
+  // Negative events *can* be recorded, but sum should not be filled out when
+  // doing so.  This is specifically to enforce compatibility w/ OpenMetrics,
+  // see: https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#histogram
+  double sum = 5;
   
   // scale describes the resolution of the histogram.  Boundaries are
   // located at powers of the base, where:
@@ -499,14 +527,14 @@ message ExponentialHistogramDataPoint {
   fixed64 zero_count = 7;
 
   // positive carries the positive range of exponential bucket counts.
-  Span positive = 8;
+  Buckets positive = 8;
 
   // negative carries the negative range of exponential bucket counts.
-  Span negative = 9;
+  Buckets negative = 9;
 
   // Buckets are a set of bucket counts, encoded in a contiguous array
   // of counts.
-  message Span {
+  message Buckets {
     // Offset is the index of the first count expressed in this span.
     // 
     // Note: This uses a varint encoding as a simple form of compression.

From 962dc00296419b63119f5a68d945e442b444a88e Mon Sep 17 00:00:00 2001
From: Joshua MacDonald <jmacd@lightstep.com>
Date: Mon, 16 Aug 2021 17:31:58 -0700
Subject: [PATCH 09/13] add to the Metric oneof

---
 opentelemetry/proto/metrics/v1/metrics.proto | 1 +
 1 file changed, 1 insertion(+)

diff --git a/opentelemetry/proto/metrics/v1/metrics.proto b/opentelemetry/proto/metrics/v1/metrics.proto
index 1c6b269ed..6939d8cae 100644
--- a/opentelemetry/proto/metrics/v1/metrics.proto
+++ b/opentelemetry/proto/metrics/v1/metrics.proto
@@ -176,6 +176,7 @@ message Metric {
     // This field will be removed in ~3 months, on July 1, 2021.
     IntHistogram int_histogram = 8 [deprecated = true];
     Histogram histogram = 9;
+    ExponentialHistogram exponential_histogram = 12;
     Summary summary = 11;
   }
 }

From 6effa0faf4d34d595abd996ee7f891f6b007ed36 Mon Sep 17 00:00:00 2001
From: Joshua MacDonald <jmacd@lightstep.com>
Date: Fri, 10 Sep 2021 16:18:09 -0700
Subject: [PATCH 10/13] Feedback applied.

---
 opentelemetry/proto/metrics/v1/metrics.proto | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/opentelemetry/proto/metrics/v1/metrics.proto b/opentelemetry/proto/metrics/v1/metrics.proto
index c2a11b4f7..6d6f26fcc 100644
--- a/opentelemetry/proto/metrics/v1/metrics.proto
+++ b/opentelemetry/proto/metrics/v1/metrics.proto
@@ -534,7 +534,7 @@ message ExponentialHistogramDataPoint {
   // Buckets are a set of bucket counts, encoded in a contiguous array
   // of counts.
   message Buckets {
-    // Offset is the index of the first count expressed in this span.
+    // Offset is the bucket index of the first entry in the bucket_counts array.
     // 
     // Note: This uses a varint encoding as a simple form of compression.
     sint32 offset = 1;
@@ -545,9 +545,9 @@ message ExponentialHistogramDataPoint {
     // base^(offset+i+1).
     //
     // Note: By contrast, the explicit HistogramDataPoint uses
-    // fixedt64.  This field is expected to have many buckets,
-    // especially zeros, so has been selected to ensure varint
-    // encoding.
+    // fixed64.  This field is expected to have many buckets,
+    // especially zeros, so uint64 has been selected to ensure
+    // varint encoding.
     repeated uint64 bucket_counts = 2;
   } 
 

From 57291833510d673b397d0f2d01af2bfff6e2e5cf Mon Sep 17 00:00:00 2001
From: Joshua MacDonald <jmacd@lightstep.com>
Date: Tue, 14 Sep 2021 16:00:16 -0700
Subject: [PATCH 11/13] update count field comment

---
 opentelemetry/proto/metrics/v1/metrics.proto | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/opentelemetry/proto/metrics/v1/metrics.proto b/opentelemetry/proto/metrics/v1/metrics.proto
index 6d6f26fcc..5886b063f 100644
--- a/opentelemetry/proto/metrics/v1/metrics.proto
+++ b/opentelemetry/proto/metrics/v1/metrics.proto
@@ -416,8 +416,7 @@ message HistogramDataPoint {
   fixed64 count = 4;
 
   // sum of the values in the population. If count is zero then this field
-  // must be zero. This value must be equal to the sum of the "sum" fields in
-  // buckets if a histogram is provided.
+  // must be zero.
   //
   // Note: Sum should only be filled out when measuring non-negative discrete
   // events, and is assumed to be monotonic over the values of these events.
@@ -482,14 +481,13 @@ message ExponentialHistogramDataPoint {
   // 1970.
   fixed64 time_unix_nano = 3;
 
-  // count is the number of values in the population. Must be non-negative. This
-  // value must be equal to the sum of the "count" fields in buckets if a
-  // histogram is provided.
+  // count is the number of values in the population. Must be
+  // non-negative. This value must be equal to the sum of the "bucket_counts"
+  // values in the positive and negative Buckets plus the "zero_count" field.
   fixed64 count = 4;
 
   // sum of the values in the population. If count is zero then this field
-  // must be zero. This value must be equal to the sum of the "sum" fields in
-  // buckets if a histogram is provided.
+  // must be zero.
   //
   // Note: Sum should only be filled out when measuring non-negative discrete
   // events, and is assumed to be monotonic over the values of these events.

From 274daa6d121b026ad0c5c386a9b21fe3e23507ef Mon Sep 17 00:00:00 2001
From: Joshua MacDonald <jmacd@lightstep.com>
Date: Tue, 14 Sep 2021 16:01:48 -0700
Subject: [PATCH 12/13] reorder top-level points; use 10

---
 opentelemetry/proto/metrics/v1/metrics.proto | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/opentelemetry/proto/metrics/v1/metrics.proto b/opentelemetry/proto/metrics/v1/metrics.proto
index 5886b063f..2592c07a0 100644
--- a/opentelemetry/proto/metrics/v1/metrics.proto
+++ b/opentelemetry/proto/metrics/v1/metrics.proto
@@ -176,7 +176,7 @@ message Metric {
     // This field will be removed in ~3 months, on July 1, 2021.
     IntHistogram int_histogram = 8 [deprecated = true];
     Histogram histogram = 9;
-    ExponentialHistogram exponential_histogram = 12;
+    ExponentialHistogram exponential_histogram = 10;
     Summary summary = 11;
   }
 }

From ba60251c5ae2d203b91b25bb6587ed5097d2132b Mon Sep 17 00:00:00 2001
From: Joshua MacDonald <jmacd@lightstep.com>
Date: Tue, 14 Sep 2021 16:02:37 -0700
Subject: [PATCH 13/13] detailed->detiled

---
 opentelemetry/proto/metrics/v1/metrics.proto | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/opentelemetry/proto/metrics/v1/metrics.proto b/opentelemetry/proto/metrics/v1/metrics.proto
index 2592c07a0..687fbfd17 100644
--- a/opentelemetry/proto/metrics/v1/metrics.proto
+++ b/opentelemetry/proto/metrics/v1/metrics.proto
@@ -344,7 +344,7 @@ message NumberDataPoint {
   repeated opentelemetry.proto.common.v1.StringKeyValue labels = 1 [deprecated = true];
 
   // StartTimeUnixNano is optional but strongly encouraged, see the
-  // the detiled comments above Metric.
+  // the detailed comments above Metric.
   //
   // Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January
   // 1970.
@@ -398,7 +398,7 @@ message HistogramDataPoint {
   repeated opentelemetry.proto.common.v1.StringKeyValue labels = 1 [deprecated = true];
 
   // StartTimeUnixNano is optional but strongly encouraged, see the
-  // the detiled comments above Metric.
+  // the detailed comments above Metric.
   //
   // Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January
   // 1970.
@@ -469,7 +469,7 @@ message ExponentialHistogramDataPoint {
   repeated opentelemetry.proto.common.v1.KeyValue attributes = 1;
 
   // StartTimeUnixNano is optional but strongly encouraged, see the
-  // the detiled comments above Metric.
+  // the detailed comments above Metric.
   //
   // Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January
   // 1970.
@@ -576,7 +576,7 @@ message SummaryDataPoint {
   repeated opentelemetry.proto.common.v1.StringKeyValue labels = 1 [deprecated = true];
 
   // StartTimeUnixNano is optional but strongly encouraged, see the
-  // the detiled comments above Metric.
+  // the detailed comments above Metric.
   //
   // Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January
   // 1970.
@@ -687,7 +687,7 @@ message IntDataPoint {
   repeated opentelemetry.proto.common.v1.StringKeyValue labels = 1;
 
   // StartTimeUnixNano is optional but strongly encouraged, see the
-  // the detiled comments above Metric.
+  // the detailed comments above Metric.
   //
   // Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January
   // 1970.
@@ -736,7 +736,7 @@ message IntHistogramDataPoint {
   repeated opentelemetry.proto.common.v1.StringKeyValue labels = 1;
 
   // StartTimeUnixNano is optional but strongly encouraged, see the
-  // the detiled comments above Metric.
+  // the detailed comments above Metric.
   //
   // Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January
   // 1970.