[Track Stats API] The stats API should be synchronous

[henbos]

In response to #98, background discussion there or this Virtual Interim recording from when it was discussed.

The performance concern I raised earlier about post tasks raised earlier are not valid, so if people see value in a sync getter API and it is easy to implement, we should do that.

Implementations that does not PostTask-update the counters just need to implement a cache instead as to run-to-completion principles.

I'll prepare a PR, then we can decide at TPAC.

[dontcallmedom-bot]

This issue was mentioned in WEBRTCWG-2023-09-12 (Page 55)