Implement RTCOutboundRtpStreamStats.targetBitrate for audio.

Spec: https://w3c.github.io/webrtc-stats/#dom-rtcoutboundrtpstreamstats-targetbitrate

Bug: webrtc:13377
Change-Id: I98dd263e0b9d6e2ca94969d2a91857b14cd65f70
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/237402
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Minyue Li <minyue@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Commit-Queue: Jakob Ivarsson <jakobi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35337}
diff --git a/api/stats/rtcstats_objects.h b/api/stats/rtcstats_objects.h
index 7742538..3795d79 100644
--- a/api/stats/rtcstats_objects.h
+++ b/api/stats/rtcstats_objects.h
@@ -529,7 +529,7 @@
   RTCStatsMember<uint64_t> bytes_sent;
   RTCStatsMember<uint64_t> header_bytes_sent;
   RTCStatsMember<uint64_t> retransmitted_bytes_sent;
-  // TODO(hbos): Collect and populate this value. https://bugs.webrtc.org/7066
+  // TODO(https://crbug.com/webrtc/13394): Also collect this metric for video.
   RTCStatsMember<double> target_bitrate;
   RTCStatsMember<uint32_t> frames_encoded;
   RTCStatsMember<uint32_t> key_frames_encoded;