feat(webrtc): add StreamCount() for metrics snapshot

app/webrtc/subsystem.go

@@ -12,8 +12,7 @@ import (
 
 // Subsystem is the app-level WebRTC egress manager. It sits alongside
 // api.API as a sibling — both consume the Restream service, both wire
-// themselves into the Echo HTTP router. The subsystem is responsible
-// for:
+// themselves into the Echo HTTP router. The subsystem is responsible for:
 //
 //   - Translating the global config.DataWebRTC into the core-level
 //     corewebrtc.Config used by the PeerFactory.
@@ -129,6 +128,14 @@ func (s *Subsystem) SetTeardownHook(fn func(streamID string)) {
 	s.teardown = fn
 }
 
+// StreamCount returns the number of processes currently registered with
+// active WebRTC egress. Used by the Prometheus snapshot collector.
+func (s *Subsystem) StreamCount() int {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return len(s.streams)
+}
+
 // lookup returns the per-process stream pair for id, or nil, false.
 // Used by the WHEP handler.
 func (s *Subsystem) lookup(id string) (*processStream, bool) {