add gauge spl_active_clients

splitter/metrics.go

···
 	Name: "spl_events_sent_counter",
 	Help: "The total number of events sent to consumers",
 }, []string{"remote_addr", "user_agent"})
+
+var activeClientGauge = promauto.NewGauge(prometheus.GaugeOpts{
+	Name: "spl_active_clients",
+	Help: "Current number of active clients",
+})

splitter/splitter.go

···
 		"cursor", since,
 		"consumer_id", consumerID,
 	)
+	activeClientGauge.Inc()
+	defer activeClientGauge.Dec()
 
 	for {
 		select {