rtc: emit per-data-track bytes via BytesTrackStats

[paulwe]

Summary

• 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.
• Wires DataTrack (UPSTREAM, at the publisher's home node where HandlePacket fires) and DataDownTrack (DOWNSTREAM, per local subscriber, at WritePacket post-marshal) into BytesTrackStats on the established 5s cadence.
• Mirrors the media-track convention: DOWNSTREAM uses the subscriber's country/ID and the publisher's track ID; UPSTREAM uses the publisher's. BytesTrackStats for DOWNSTREAM is constructed inside DataTrack.AddSubscriber from the subscriber's LocalParticipant accessors; UPSTREAM is constructed at the two publisher-home NewDataTrack call sites (HandlePublishDataTrackRequest and the migration path).
• Cross-region proxy DataTrack instances (e.g. cloud's RemoteParticipant) leave the new params field nil — they have no publisher reporter on that node, and relayed bytes routed through HandlePacket would double-count.
• Legacy dataTrackStats packet-loss / out-of-order / frame counters are preserved unchanged.

Test plan

• go test -short ./pkg/rtc/... passes
• Cloud workspace builds + go test -short ./pkg/rtc/... passes (no cloud edits needed — additive params field defaults to nil at proxy and replay sites)
• End-to-end: publish a data track + subscribe, confirm OnTrackStats receives both StatsKeyForData(UPSTREAM, publisherID, dataTrackID) (publisher node) and StatsKeyForData(DOWNSTREAM, subscriberID, dataTrackID) (subscriber node) on ~5s cadence with a final flush on Close
• Cross-region replay: confirm proxy DataTrack on subscriber node emits no UPSTREAM (no nil panic, no double accounting)

[anunaym14]

livekit/pkg/rtc/participant.go

Line 291 in 2e22911

dataChannelStats *BytesTrackStats

this probably needs to be removed?