util/usermetrics: make usermetrics non-global

this commit changes usermetrics to be non-global, this is a building
block for correct metrics if a go process runs multiple tsnets or
in tests.

Updates #13420
Updates tailscale/corp#22075

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

ipn/ipnlocal/local.go

@@ -119,9 +119,6 @@ import (
 	"tailscale.com/wgengine/wgcfg/nmcfg"
 )
 
-var metricAdvertisedRoutes = usermetric.NewGauge(
-	"tailscaled_advertised_routes", "Number of advertised network routes (e.g. by a subnet router)")
-
 var controlDebugFlags = getControlDebugFlags()
 
 func getControlDebugFlags() []string {
@@ -184,6 +181,7 @@ type LocalBackend struct {
 	statsLogf             logger.Logf        // for printing peers stats on change
 	sys                   *tsd.System
 	health                *health.Tracker // always non-nil
+	metrics               metrics
 	e                     wgengine.Engine // non-nil; TODO(bradfitz): remove; use sys
 	store                 ipn.StateStore  // non-nil; TODO(bradfitz): remove; use sys
 	dialer                *tsdial.Dialer  // non-nil; TODO(bradfitz): remove; use sys
@@ -377,6 +375,11 @@ func (b *LocalBackend) HealthTracker() *health.Tracker {
 	return b.health
 }
 
+// UserMetricsRegistry returns the usermetrics registry for the backend
+func (b *LocalBackend) UserMetricsRegistry() *usermetric.Registry {
+	return b.sys.UserMetricsRegistry()
+}
+
 // NetMon returns the network monitor for the backend.
 func (b *LocalBackend) NetMon() *netmon.Monitor {
 	return b.sys.NetMon.Get()
@@ -386,6 +389,12 @@ type updateStatus struct {
 	started bool
 }
 
+type metrics struct {
+	// advertisedRoutes is a metric that counts the number of network routes that are advertised by the local node.
+	// This informs the user of how many routes are being advertised by the local node, excluding exit routes.
+	advertisedRoutes *usermetric.Gauge
+}
+
 // clientGen is a func that creates a control plane client.
 // It's the type used by LocalBackend.SetControlClientGetterForTesting.
 type clientGen func(controlclient.Options) (controlclient.Client, error)
@@ -429,6 +438,11 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, sys *tsd.System, lo
 	captiveCtx, captiveCancel := context.WithCancel(ctx)
 	captiveCancel()
 
+	m := metrics{
+		advertisedRoutes: sys.UserMetricsRegistry().NewGauge(
+			"tailscaled_advertised_routes", "Number of advertised network routes (e.g. by a subnet router)"),
+	}
+
 	b := &LocalBackend{
 		ctx:                   ctx,
 		ctxCancel:             cancel,
@@ -437,6 +451,7 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, sys *tsd.System, lo
 		statsLogf:             logger.LogOnChange(logf, 5*time.Minute, clock.Now),
 		sys:                   sys,
 		health:                sys.HealthTracker(),
+		metrics:               m,
 		e:                     e,
 		dialer:                dialer,
 		store:                 store,
@@ -4760,7 +4775,7 @@ func (b *LocalBackend) applyPrefsToHostinfoLocked(hi *tailcfg.Hostinfo, prefs ip
 			routes++
 		}
 	}
-	metricAdvertisedRoutes.Set(float64(routes))
+	b.metrics.advertisedRoutes.Set(float64(routes))
 
 	var sshHostKeys []string
 	if prefs.RunSSH() && envknob.CanSSHD() {