Add optional min and max fields to histogram data model. (#1915)

* Add optional min and max fields to histogram data model.

* Fix lint error

* Update histogram min and max definition per discussion

* Format changelog to comply with line length recommendations

* Update delta histogram image with min and max

* Fix lint error

* Revert "Fix lint error"

This reverts commit 90f1c72

* Removing trailing space on line

Co-authored-by: Josh Suereth <[email protected]>

CHANGELOG.md

@@ -13,6 +13,9 @@ release.
 
 ### Metrics
 
+- Add optional min / max fields to histogram data model.
+  ([#1915](https://github.com/open-telemetry/opentelemetry-specification/pull/1915))
+
 ### Logs
 
 ### Resource

specification/metrics/datamodel.md

@@ -386,6 +386,8 @@ Histograms consist of the following:
       (00:00:00 UTC on 1 January 1970).
   - A count (`count`) of the total population of points in the histogram.
   - A sum (`sum`) of all the values in the histogram.
+  - (optional) The min (`min`) of all values in the histogram.
+  - (optional) The max (`max`) of all values in the histogram.
   - (optional) A series of buckets with:
     - Explicit boundary values.  These values denote the lower and upper bounds
       for buckets and whether not a given observation would be recorded in this
@@ -398,6 +400,13 @@ denotes Delta temporality where accumulated event counts are reset to zero after
 and a new aggregation occurs. Cumulative, on the other hand, continues to
 aggregate events, resetting with the use of a new start time.
 
+The aggregation temporality also has implications on the min and max fields. Min
+and max are more useful for Delta temporality, since the values represented by
+Cumulative min and max will stabilize as more events are recorded. Additionally,
+it is possible to convert min and max from Delta to Cumulative, but not from
+Cumulative to Delta. When converting from Cumulative to Delta, min and max can
+be dropped, or captured in an alternative representation such as a gauge.
+
 Bucket counts are optional.  A Histogram without buckets conveys a
 population in terms of only the sum and count, and may be interpreted
 as a histogram with single bucket covering `(-Inf, +Inf)`.