Expose the usefull experimental stats to a JavaScript layer

[Doreapp]

Some statistics, such as freezeCount, totalFreezeDuration, pauseCount and totalPauseDuration are only available from a native level or through webrtc-internals.

Those are documented as webrtc-provisional-stats and seem to be implemented some time ago (according to this commit in February 2019)

I would love to be able to access these metrics from a JavaScript level, ideally from the getStats function.

[vr000m]

The experimental stats should be available via getStats. In the actual browser implementation, it might be available via origin-trials. @henbos can throw some light on the status of the Freeze* metrics in Chrome.

[henbos]

We have exposed experimental metrics in the past in origin trials but I don't think these metrics are tied to any ongoing experiments afaik.

Most likely these metrics were added by native users due to "probably being useful" but not having any strong justification or backing by the spec. So either we never followed up on them after them having been proven useful or alternaively maybe they weren't useful.

But probably we just forgot about them :)

[henbos]

This was being asked for again in #695, let's close this one as a dupe, I cc'd someone on that other issue to shed some light on this. I'm not opposed to exposing these metrics