Please comment: Guidelines on how to add new getStats() metrics

I've written Guidelines on how to add new getStats() metrics
<https://docs.google.com/document/d/1q1CJVUqJ6YW9NNRc0tENkLNny8AHrKZfqjy3SL89zjc/edit?usp=sharing>
in
Chrome for the RTCPeerConnection.getStats() (API at webrtc-pc
<https://w3c.github.io/webrtc-pc/#dom-rtcpeerconnection-getstats>, stats
dictionaries at webrtc-stats <https://w3c.github.io/webrtc-stats/>).

Please take a look and comment.

My hope is that more people will dare to take the path towards
standardization and allow us to completely ban adding new stats in Chrome's
non-compliant legacy callback-based version of getStats() whose goog-stats
so many people rely on today. This should be step towards deprecating
legacy getStats() in Chrome and making spec-compliant getStats() the clear
choice going forward.