Add 'metadata' field to MetricsSet proto.

Bug: b/266997275
Change-Id: Iece033b0bd3b6e2946a57ae19dd4ff0a0417694f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/291535
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39232}
diff --git a/api/test/metrics/proto/metric.proto b/api/test/metrics/proto/metric.proto
index cdd3125..94921a5 100644
--- a/api/test/metrics/proto/metric.proto
+++ b/api/test/metrics/proto/metric.proto
@@ -15,6 +15,8 @@
 // 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 {