The WebRTC getStats API is specified in https://w3c.github.io/webrtc-stats/ and allows querying information about the current state of a RTCPeerConnection API and some of its member objects.
When adding a new standardized RTCStatsMember it is necessary to add it to the Chrome allowlist chrome/test/data/webrtc/peerconnection_getstats.js before landing the WebRTC change. This mechanism prevents the accidential addition and exposure of non-standard attributes and is not required for RTCNonStandardStatsMember which is not exposed to the web API.