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/pc/rtc_stats_collector.cc b/pc/rtc_stats_collector.cc
index 7637c07..23e24dc 100644
--- a/pc/rtc_stats_collector.cc
+++ b/pc/rtc_stats_collector.cc
@@ -536,6 +536,9 @@
                                                outbound_audio);
   outbound_audio->media_type = "audio";
   outbound_audio->kind = "audio";
+  if (voice_sender_info.target_bitrate > 0) {
+    outbound_audio->target_bitrate = voice_sender_info.target_bitrate;
+  }
   if (voice_sender_info.codec_payload_type) {
     outbound_audio->codec_id = RTCCodecStatsIDFromMidDirectionAndPayload(
         mid, /*inbound=*/false, *voice_sender_info.codec_payload_type);