Move MediaStreamTrack stats in its own spec?

[dontcallmedom]

With the document in this repo being a mix of ideas with different levels of support, implementation and maintenance, I think we should only add new features in it if there isn't a better place.

The default would be to add them in mediacatpure-main, but presumably we would only want to do that once we have double implementation to avoid creating features at risk in the current CR (although maybe that wouldn't be so bad given that they could be be kept as "candidate additions" in a possible future Rec).

But when we have features that layer clearly on top of mediacapture-main (e.g. don't need monkey patching algorithms), a reasonably simple approach would be to publish them as independent spec. This has other benefits - clearly separating the relevant tests in WPT, clearly separating the relevant issues in a repo of their own; simplifying their exposure in webref and its downstream ecosystem (MDN, BCD, WPT, …)

This seems to apply to the recent addition of stats (some of which are starting to ship in Chromium from what I gather), and so I would suggest we move the two said sections in a spec of their own. Thoughts? I'm happy to take care of the logistics of the split, although we would need to have a committed editor.