quic: only register Prometheus counters when metrics are enabled

p2p/transport/quicreuse/connmgr.go

@@ -54,7 +54,7 @@ func NewConnManager(statelessResetKey quic.StatelessResetKey, opts ...Option) (*
 		tracers = append(tracers, qlogTracer)
 	}
 	if cm.enableMetrics {
-		tracers = append(tracers, &metricsTracer{})
+		tracers = append(tracers, newMetricsTracer())
 	}
 	if len(tracers) > 0 {
 		quicConf.Tracer = quiclogging.NewMultiplexedTracer(tracers...)

p2p/transport/quicreuse/tracer_metrics.go

@@ -85,7 +85,9 @@ func (c *aggregatingCollector) RemoveConn(id string) {
 
 var collector *aggregatingCollector
 
-func init() {
+var initMetricsOnce sync.Once
+
+func initMetrics() {
 	const (
 		direction = "direction"
 		encLevel  = "encryption_level"
@@ -173,6 +175,11 @@ type metricsTracer struct {
 
 var _ logging.Tracer = &metricsTracer{}
 
+func newMetricsTracer() *metricsTracer {
+	initMetricsOnce.Do(func() { initMetrics() })
+	return &metricsTracer{}
+}
+
 func (m *metricsTracer) TracerForConnection(_ context.Context, p logging.Perspective, connID logging.ConnectionID) logging.ConnectionTracer {
 	return &metricsConnTracer{perspective: p, connID: connID}
 }