Metrics: Add Exemplar to DistributionValue. (#62)

opencensus/proto/stats/metrics/metrics.proto

@@ -226,12 +226,28 @@ message DistributionValue {
     // The number of values in each bucket of the histogram, as described in
     // bucket_bounds.
     int64 count = 1;
-
-    // TODO: Add support for exemplars.
   }
 
   // If the distribution does not have a histogram, then omit this field.
   // If there is a histogram, then the sum of the values in the Bucket counts
   // must equal the value in the count field of the distribution.
   repeated Bucket buckets = 6;
+
+  // Exemplars are example points that may be used to annotate aggregated
+  // Distribution values. They are metadata that gives information about a
+  // particular value added to a Distribution bucket.
+  message Exemplar {
+    // Value of the exemplar point. It determines which bucket the exemplar
+    // belongs to.
+    double value = 1;
+
+    // The observation (sampling) time of the above value.
+    google.protobuf.Timestamp timestamp = 2;
+
+    // Contextual information about the example value.
+    map<string, string> attachments = 3;
+  }
+
+  // If the distribution does not have a histogram, then omit this field.
+  repeated Exemplar exemplars = 7;
 }