[webrtc-stats] TimingInfo metrics (#704)

henbos has just created a new issue for https://github.com/w3c/webrtc-stats:

== TimingInfo metrics ==
In Chrome, the legacy getStats() contains this non-standard metric: `googTimingFrameInfo`.

Apparently it is very useful. It is exposed as [a string that has to be parsed](https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/api/video/video_timing.cc;l=82;drc=8d399817282e3c12ed54eb23ec42a5e418298ec6) rather than the following attributes:
- RTP timestamp
- Capture time
- Encode start
- Encode finish
- Packetization finish
- Pacer exit
- Network timestamp
- Receive start
- Receive finish
- Decode start
- Decode finish
- Render time
- Boolean for "is outlier"
- Boolean for "triggered by timer"

I supposed it is one serialized struct because all of these metrics relate to a single video frame. The modern way to do this would be to create a new RTCStatsType and have one stats object containing all of these attributes, then reference that frame object from the "inbound-rtp".

I can guess what some of these are but all of them needs a precise definition. @ilyanikolaevskiy Can you enlighten us?

Please view or discuss this issue at https://github.com/w3c/webrtc-stats/issues/704 using your GitHub account


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