add session start time metric (#2377)

pkg/rtc/participant.go

@@ -97,6 +97,7 @@ type ParticipantParams struct {
 	AudioConfig             config.AudioConfig
 	VideoConfig             config.VideoConfig
 	ProtocolVersion         types.ProtocolVersion
+	SessionStartTime        time.Time
 	Telemetry               telemetry.TelemetryService
 	Trailer                 []byte
 	PLIThrottleConfig       config.PLIThrottleConfig
@@ -1421,6 +1422,7 @@ func (p *ParticipantImpl) onPrimaryTransportInitialConnected() {
 }
 
 func (p *ParticipantImpl) onPrimaryTransportFullyEstablished() {
+	prometheus.RecordSessionStartTime(int(p.ProtocolVersion()), time.Since(p.params.SessionStartTime))
 	p.updateState(livekit.ParticipantInfo_ACTIVE)
 }
 

pkg/rtc/participant_internal_test.go

@@ -753,6 +753,7 @@ func newParticipantForTestWithOpts(identity livekit.ParticipantIdentity, opts *p
 		Config:                 rtcConf,
 		Sink:                   &routingfakes.FakeMessageSink{},
 		ProtocolVersion:        opts.protocolVersion,
+		SessionStartTime:       time.Now(),
 		PLIThrottleConfig:      conf.RTC.PLIThrottle,
 		Grants:                 grants,
 		PublishEnabledCodecs:   enabledCodecs,

pkg/service/roommanager.go

@@ -245,6 +245,8 @@ func (r *RoomManager) StartSession(
 	requestSource routing.MessageSource,
 	responseSink routing.MessageSink,
 ) error {
+	sessionStartTime := time.Now()
+
 	room, err := r.getOrCreateRoom(ctx, roomName)
 	if err != nil {
 		return err
@@ -390,6 +392,7 @@ func (r *RoomManager) StartSession(
 		AudioConfig:             r.config.Audio,
 		VideoConfig:             r.config.Video,
 		ProtocolVersion:         pv,
+		SessionStartTime:        sessionStartTime,
 		Telemetry:               r.telemetry,
 		Trailer:                 room.Trailer(),
 		PLIThrottleConfig:       r.config.RTC.PLIThrottle,

pkg/telemetry/prometheus/rooms.go

@@ -15,6 +15,7 @@
 package prometheus
 
 import (
+	"strconv"
 	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
@@ -43,6 +44,7 @@ var (
 	promTrackSubscribedCurrent *prometheus.GaugeVec
 	promTrackPublishCounter    *prometheus.CounterVec
 	promTrackSubscribeCounter  *prometheus.CounterVec
+	promSessionStartTime       prometheus.HistogramVec
 )
 
 func initRoomStats(nodeID string, nodeType livekit.NodeType, env string) {
@@ -91,6 +93,13 @@ func initRoomStats(nodeID string, nodeType livekit.NodeType, env string) {
 		Name:        "subscribe_counter",
 		ConstLabels: prometheus.Labels{"node_id": nodeID, "node_type": nodeType.String(), "env": env},
 	}, []string{"state", "error"})
+	promSessionStartTime = *prometheus.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace:   livekitNamespace,
+		Subsystem:   "session",
+		Name:        "start_time_ms",
+		ConstLabels: prometheus.Labels{"node_id": nodeID, "node_type": nodeType.String(), "env": env},
+		Buckets:     prometheus.ExponentialBucketsRange(100, 5000, 10),
+	}, []string{"protocol_version"})
 
 	prometheus.MustRegister(promRoomCurrent)
 	prometheus.MustRegister(promRoomDuration)
@@ -99,6 +108,7 @@ func initRoomStats(nodeID string, nodeType livekit.NodeType, env string) {
 	prometheus.MustRegister(promTrackSubscribedCurrent)
 	prometheus.MustRegister(promTrackPublishCounter)
 	prometheus.MustRegister(promTrackSubscribeCounter)
+	prometheus.MustRegister(promSessionStartTime)
 }
 
 func RoomStarted() {
@@ -172,3 +182,7 @@ func RecordTrackSubscribeFailure(err error, isUserError bool) {
 		trackSubscribeUserError.Inc()
 	}
 }
+
+func RecordSessionStartTime(protocolVersion int, d time.Duration) {
+	promSessionStartTime.WithLabelValues(strconv.Itoa(protocolVersion)).Observe(float64(d.Milliseconds()))
+}