add gauge bgs_connected_inbound

bgs/fedmgr.go

···
 
 	cursor := host.Cursor
 
+	connectedInbound.Inc()
+	defer connectedInbound.Dec()
+	// TODO:? maybe keep a gauge of 'in retry backoff' sources?
+
 	var backoff int
 	for {
 		select {

bgs/metrics.go

···
 	Help: "The total number of external users created",
 })
 
+var connectedInbound = promauto.NewGauge(prometheus.GaugeOpts{
+	Name: "bgs_connected_inbound",
+	Help: "Number of inbound firehoses we are consuming",
+})
+
 var compactionDuration = promauto.NewHistogram(prometheus.HistogramOpts{
 	Name:    "compaction_duration",
 	Help:    "A histogram of compaction latencies",