Introduce GN arg rtc_exclude_metrics_default.

This GN argument will be used to exclude the default implementation of
metrics in order to allow clients to provide a custom implementation.

This will allow to land [1] without breaking Chromium.

[1] - https://webrtc-review.googlesource.com/c/src/+/98102

Bug: webrtc:9631
Change-Id: I035a12a1b4985048272b6b849a3a9fb9d66ed8b5
Reviewed-on: https://webrtc-review.googlesource.com/95428
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24585}

webrtc.gni

@@ -46,6 +46,14 @@ declare_args() {
     rtc_exclude_field_trial_default = false
   }
 
+  # WARNING: This argument doesn't have any effect on the WebRTC build until
+  # https://webrtc-review.googlesource.com/c/src/+/95480 will land.
+  # Setting this to true will define WEBRTC_EXCLUDE_METRICS_DEFAULT which
+  # will tell the pre-processor to remove the default definition of symbols
+  # needed to use metrics. In that case a new implementation needs to be
+  # provided.
+  rtc_exclude_metrics_default = build_with_chromium
+
   # Setting this to false will require the API user to pass in their own
   # SSLCertificateVerifier to verify the certificates presented from a
   # TLS-TURN server. In return disabling this saves around 100kb in the binary.