Optimizations to histogram protobuf encoding.

components/metrics/proto/histogram_event.proto

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Histogram-collected metrics.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 option java_outer_classname = "HistogramEventProtos";
 option java_package = "org.chromium.components.metrics";
 
 package metrics;
 
 // Next tag: 4
 message HistogramEventProto {
   // The name of the histogram, hashed.
   optional fixed64 name_hash = 1;
 
   // 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.
   message Bucket {
     // Each bucket's range is bounded by min <= x < max.
     // It is valid to omit one of these two fields in a bucket, but not both.
     // If the min field is omitted, its value is assumed to be equal to max - 1.
     // If the max field is omitted, its value is assumed to be equal to the next
     // bucket's min value (possibly computed per above).  The last bucket in a
     // histogram should always include the max field.
     optional int64 min = 1;
     optional int64 max = 2;
 
     // The bucket's index in the list of buckets, sorted in ascending order.
     // This field was intended to provide extra redundancy to detect corrupted
     // records, but was never used.  As of M31, it is no longer sent by Chrome
     // 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;
 }