Optimizations to histogram protobuf encoding.

Omits several fields from being sent if their value is one of the
common field values.

BUG=590765

Review URL: https://codereview.chromium.org/1744283002

Cr-Commit-Position: refs/heads/master@{#378364}

components/metrics/histogram_encoder.cc

@@ -25,7 +25,8 @@ void EncodeHistogramDelta(const std::string& histogram_name,
 
   HistogramEventProto* histogram_proto = uma_proto->add_histogram_event();
   histogram_proto->set_name_hash(base::HashMetricName(histogram_name));
-  histogram_proto->set_sum(snapshot.sum());
+  if (snapshot.sum() != 0)
+    histogram_proto->set_sum(snapshot.sum());
 
   for (scoped_ptr<SampleCountIterator> it = snapshot.Iterator(); !it->Done();
        it->Next()) {
@@ -36,7 +37,9 @@ void EncodeHistogramDelta(const std::string& histogram_name,
     HistogramEventProto::Bucket* bucket = histogram_proto->add_bucket();
     bucket->set_min(min);
     bucket->set_max(max);
-    bucket->set_count(count);
+    // Note: The default for count is 1 in the proto, so omit it in that case.
+    if (count != 1)
+      bucket->set_count(count);
   }
 
   // Omit fields to save space (see rules in histogram_event.proto comments).

components/metrics/proto/histogram_event.proto

@@ -20,7 +20,8 @@ message HistogramEventProto {
   // The sum of all the sample values.
   // Together with the total count of the sample values, this allows us to
   // compute the average value.  The count of all sample values is just the sum
-  // of the counts of all the buckets.
+  // of the counts of all the buckets.  As of M51, when the value of this field
+  // would be 0, the field will be omitted instead.
   optional int64 sum = 2;
 
   // The per-bucket data.
@@ -40,8 +41,9 @@ message HistogramEventProto {
     // clients to reduce the UMA upload size.
     optional int32 bucket_index = 3 [deprecated = true];
 
-    // The number of entries in this bucket.
-    optional int64 count = 4;
+    // The number of entries in this bucket.  As of M51, when the value of this
+    // field would be 1, the field will be omitted instead.
+    optional int64 count = 4 [default = 1];
   }
   repeated Bucket bucket = 3;
 }