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}

modules/audio_processing/rms_level.h

@@ -34,7 +34,7 @@ class RmsLevel {
     int peak;
   };
 
-  static constexpr int kMinLevelDb = 127;
+  enum : int { kMinLevelDb = 127 };
 
   RmsLevel();
   ~RmsLevel();