Add flag to export in new MetricSet proto format

Bug: b/246095034
Change-Id: I7e879ab9d47267788793a65a75fba401bf9aa38a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/277561
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38259}
diff --git a/test/test_flags.h b/test/test_flags.h
index 694b454..6ca30b2 100644
--- a/test/test_flags.h
+++ b/test/test_flags.h
@@ -18,5 +18,6 @@
 ABSL_DECLARE_FLAG(std::string, force_fieldtrials);
 ABSL_DECLARE_FLAG(std::vector<std::string>, plot);
 ABSL_DECLARE_FLAG(std::string, isolated_script_test_perf_output);
+ABSL_DECLARE_FLAG(std::string, webrtc_test_metrics_output_path);
 
 #endif  // TEST_TEST_FLAGS_H_