Metric: Move label keys to metric descriptor. (#55)

Author: songy23

opencensus/proto/stats/metrics/metrics.proto

@@ -69,15 +69,19 @@ 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;
 }
 
 // A collection of data points that describes the time-varying values
 // of a metric.
 message TimeSeries {
+  // TODO: Wrap values to distinguish between empty-string and unset.
   // TODO: Add restrictions for characters that can be used for keys and values.
   // The set of label values that uniquely identify this timeseries. Apply to all
-  // points.
-  map<string, string> label = 1;
+  // points. The order of label values must match that of label keys in the
+  // metric descriptor.
+  repeated string label_values = 1;
 
   // The data points of this timeseries. Point type MUST match the MetricDescriptor.type, so for
   // a CUMULATIVE type only cumulative_points are present and for a GAUGE type only gauge_points