Metric: Add LabelKey type that has key and description. (#57)

Author: songy23

opencensus/proto/stats/metrics/metrics.proto

@@ -75,9 +75,17 @@ message MetricDescriptor {
   }
   Type type = 4;
 
-  // TODO: Consider to add LabelInfo{key, description} if needed.
   // The label keys associated with the metric descriptor.
-  repeated string label_keys = 5;
+  repeated LabelKey label_keys = 5;
+}
+
+// Defines a label key associated with a metric descriptor.
+message LabelKey {
+  // The key for the label.
+  string key = 1;
+
+  // A human-readable description of what this label key represents.
+  string description = 2;
 }
 
 // A collection of data points that describes the time-varying values