Optimizations to histogram protobuf encoding.

components/metrics/proto/histogram_event.proto

Index: components/metrics/proto/histogram_event.proto
diff --git a/components/metrics/proto/histogram_event.proto b/components/metrics/proto/histogram_event.proto
index 4b68094ff6dfab3fb6a44861753e57a09ee18f7d..8b054172eae91eabc5229a4f8793d9274513c73b 100644
--- a/components/metrics/proto/histogram_event.proto
+++ b/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;
 }