Extend webrtc_session Prometheus metrics (bitrate, jitter, packageloss)

[yelodevopsi]

Describe the feature

• Today we only have total amount of packages sent and received in the stats.
• I'd like to at least have the bitrate extracted. Bonus for jitter and packageloss (and all other chrome://webrtc-internals equivalent params)
• I see the SRT protocol supports this, but I also see this work was a import of an impressive srt-lib 👏

I'm not sure if there already exists an webrtc stats object in the code somewhere which it might be easy to implement this?