Add directive to make webrtc metrics optional.

Bug: webrtc:11144
Change-Id: I4e75e6aec033784685de3670e880bb9f2b6ee8d2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/161043
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Ying Wang <yinwa@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30040}
diff --git a/webrtc.gni b/webrtc.gni
index 56a1b0d..513d5fc 100644
--- a/webrtc.gni
+++ b/webrtc.gni
@@ -252,6 +252,9 @@
   # Set this to true to disable detailed error message and logging for
   # RTC_CHECKs.
   rtc_disable_check_msg = false
+
+  # Set this to true to disable webrtc metrics.
+  rtc_disable_metrics = true
 }
 
 # Make it possible to provide custom locations for some libraries (move these