Recording Duration and StartTime (#1280)

* recording header duration and start time

* generated protobuf

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Co-authored-by: github-actions <41898282+github-actions[bot]@users.noreply.github.com>

Author: barrust

livekit/livekit_metrics.pb.go

@@ -91,4 +91,6 @@ message EventMetric {
 message MetricsRecordingHeader {
   string room_id = 1;
   optional bool enable_user_data_training = 2;
+  uint64 duration = 3; // milliseconds
+  google.protobuf.Timestamp start_time = 4;
 }