Add AudioProcessingCaptureStats and a level estimator replacement

This adds an interface for accessing stats on the capture stream, and
adds a level estimator to report one of the stats.

Bug: webrtc:9947
Change-Id: Id472534fa2e04d46c9ab700671f620584a246afb
Reviewed-on: https://webrtc-review.googlesource.com/c/109587
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25786}

modules/audio_processing/include/audio_processing_statistics.h

@@ -24,6 +24,14 @@ struct RTC_EXPORT AudioProcessingStats {
   AudioProcessingStats(const AudioProcessingStats& other);
   ~AudioProcessingStats();
 
+  // The root mean square (RMS) level in dBFS (decibels from digital
+  // full-scale) of the last capture frame, after processing. It is
+  // constrained to [-127, 0].
+  // The computation follows: https://tools.ietf.org/html/rfc6465
+  // with the intent that it can provide the RTP audio level indication.
+  // Only reported if level estimation is enabled in AudioProcessing::Config.
+  absl::optional<int> output_rms_dbfs;
+
   // AEC Statistics.
   // ERL = 10log_10(P_far / P_echo)
   absl::optional<double> echo_return_loss;