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}