Data tracks (the new _data_track datachannel) previously only updated a
private dataTrackStats that logged a single summary at Close. Bytes never
reached the OnTrackStats -> TelemetryService.TrackStats pipeline that
media tracks and signal channels feed.
Wire DataTrack (UPSTREAM, publisher-home) and DataDownTrack (DOWNSTREAM,
per-subscriber) into BytesTrackStats on the same 5s cadence, mirroring
the media-track convention: subscriber's country and ID with publisher's
track ID for DOWNSTREAM. Cross-region proxy DataTracks leave the stats
pointer nil (no publisher reporter on that node, and relayed bytes would
double-count). Legacy dataTrackStats packet-loss/frame counters are
preserved.
