Add generated comfort noise counter.

Currently only implemented for codec internal CNG (Opus).

Bug: webrtc:13322
Change-Id: I00622f2967f066dba64a792e26081038ae0cb0d9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/259200
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Commit-Queue: Jakob Ivarsson‎ <jakobi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36590}

modules/audio_coding/neteq/neteq_impl.cc

@@ -1455,6 +1455,7 @@ int NetEqImpl::DecodeCng(AudioDecoder* decoder,
       return kDecodedTooMuch;
     }
   }
+  stats_->GeneratedNoiseSamples(*decoded_length);
   return 0;
 }