ipn/ipnlocal: add client metric gauge for number of IPNBus connections

Updates #1708

Change-Id: Ic7e28d692b4c48e78c842c26234b861fe42a916e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -95,6 +95,7 @@ import (
 	"tailscale.com/types/preftype"
 	"tailscale.com/types/ptr"
 	"tailscale.com/types/views"
+	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/deephash"
 	"tailscale.com/util/dnsname"
 	"tailscale.com/util/goroutines"
@@ -2863,6 +2864,9 @@ func (b *LocalBackend) WatchNotificationsAs(ctx context.Context, actor ipnauth.A
 	mak.Set(&b.notifyWatchers, sessionID, session)
 	b.mu.Unlock()
 
+	metricCurrentWatchIPNBus.Add(1)
+	defer metricCurrentWatchIPNBus.Add(-1)
+
 	defer func() {
 		b.mu.Lock()
 		delete(b.notifyWatchers, sessionID)
@@ -7696,3 +7700,7 @@ func (b *LocalBackend) vipServicesFromPrefsLocked(prefs ipn.PrefsView) []*tailcf
 
 	return slicesx.MapValues(services)
 }
+
+var (
+	metricCurrentWatchIPNBus = clientmetric.NewGauge("localbackend_current_watch_ipn_bus")
+)