message: Add 'metadata' field to MetricsSet proto.
  Cr-Commit-Position: refs/heads/main@{#39232}
diff: @@ -15,6 +15,8 @@ package webrtc.test_metrics;
  // Root message of the proto file. Contains collection of all the metrics.
  message MetricsSet {
  repeated Metric metrics = 1;
  + // Metadata associated with the whole metrics set.
  + map<string, string> metadata = 2;
  }
  enum Unit {
  