add exemplar to native histogram (#80)

Add exemplars to native histogram

Signed-off-by: Ziqi Zhao <[email protected]>

Author: fatsheep9146

go/metrics.pb.go

@@ -107,6 +107,9 @@ message Histogram {
   // histograms.
   repeated sint64 positive_delta    = 13; // Count delta of each bucket compared to previous one (or to zero for 1st bucket).
   repeated double positive_count    = 14; // Absolute count of each bucket.
+
+  // Only used for native histograms. These exemplars MUST have a timestamp.
+  repeated Exemplar exemplars = 16;
 }
 
 // A Bucket of a conventional histogram, each of which is treated as