feat(prometheus): add WebRTC snapshot collector

prometheus/webrtc.go

@@ -0,0 +1,75 @@
+package prometheus
+
+import (
+	// Hybrid instrumentation rationale: direct client_golang instrumentation
+	// lives in app/webrtc/metrics.go (hot-path counters and histograms);
+	// this file owns the snapshot-style gauges. See the design doc at
+	// docs/design/2026-05-03-datarhei-dragon-fork-webrtc-prometheus-metrics-design.md.
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// WebRTCStats is a point-in-time snapshot of the WebRTC subsystem state.
+// Populated by Handler.Stats() in app/webrtc and consumed by the collector
+// at scrape time.
+type WebRTCStats struct {
+	StreamCount   int
+	PeersByStream map[string]int
+	UDPPortsInUse int
+}
+
+// WebRTCStatsSource is implemented by app/webrtc.Handler (via its Stats()
+// method). The interface lives here so prometheus/ does not import app/webrtc,
+// keeping the dependency arrow one-directional.
+type WebRTCStatsSource interface {
+	Stats() WebRTCStats
+}
+
+type webrtcCollector struct {
+	core   string
+	source WebRTCStatsSource
+
+	activeStreamsDesc  *prometheus.Desc
+	activePeersDesc   *prometheus.Desc
+	udpPortsInUseDesc *prometheus.Desc
+}
+
+// NewWebRTCCollector returns a prometheus.Collector that emits three gauge
+// metrics at scrape time by calling source.Stats(). Register it with the
+// shared Metrics registry before the /metrics endpoint starts serving.
+func NewWebRTCCollector(core string, source WebRTCStatsSource) prometheus.Collector {
+	cl := prometheus.Labels{"core": core}
+	return &webrtcCollector{
+		core:   core,
+		source: source,
+		activeStreamsDesc: prometheus.NewDesc(
+			"dragonfork_webrtc_active_streams",
+			"Streams currently registered (processes with webrtc.enabled=true running).",
+			nil, cl,
+		),
+		activePeersDesc: prometheus.NewDesc(
+			"dragonfork_webrtc_active_peers",
+			"Currently subscribed WHEP peers per stream.",
+			[]string{"stream_id"}, cl,
+		),
+		udpPortsInUseDesc: prometheus.NewDesc(
+			"dragonfork_webrtc_udp_ports_in_use",
+			"UDP ports currently allocated (2 per active stream).",
+			nil, cl,
+		),
+	}
+}
+
+func (c *webrtcCollector) Describe(ch chan<- *prometheus.Desc) {
+	ch <- c.activeStreamsDesc
+	ch <- c.activePeersDesc
+	ch <- c.udpPortsInUseDesc
+}
+
+func (c *webrtcCollector) Collect(ch chan<- prometheus.Metric) {
+	stats := c.source.Stats()
+	ch <- prometheus.MustNewConstMetric(c.activeStreamsDesc, prometheus.GaugeValue, float64(stats.StreamCount))
+	ch <- prometheus.MustNewConstMetric(c.udpPortsInUseDesc, prometheus.GaugeValue, float64(stats.UDPPortsInUse))
+	for streamID, count := range stats.PeersByStream {
+		ch <- prometheus.MustNewConstMetric(c.activePeersDesc, prometheus.GaugeValue, float64(count), streamID)
+	}
+}