make route a simpler expvar

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

ipn/ipnlocal/local.go

@@ -12,6 +12,7 @@
 	"encoding/base64"
 	"encoding/json"
 	"errors"
+	"expvar"
 	"fmt"
 	"io"
 	"log"
@@ -118,6 +119,12 @@
 	"tailscale.com/wgengine/wgcfg/nmcfg"
 )
 
+var metricAdvertisedRoutes expvar.Int
+
+func init() {
+	usermetric.Publish("tailscaled_advertised_routes", &metricAdvertisedRoutes)
+}
+
 var controlDebugFlags = getControlDebugFlags()
 
 func getControlDebugFlags() []string {
@@ -4615,12 +4622,6 @@ func unmapIPPrefixes(ippsList ...[]netip.Prefix) (ret []netip.Prefix) {
 	return ret
 }
 
-var metricAdvertisedRoutes = usermetric.NewMap(
-	"tailscaled_advertised_routes",
-	"gauge",
-	"Number of subnet routes advertised by the node. (excluding exit node /0 routes)",
-)
-
 // b.mu must be held.
 func (b *LocalBackend) applyPrefsToHostinfoLocked(hi *tailcfg.Hostinfo, prefs ipn.PrefsView) {
 	if h := prefs.Hostname(); h != "" {
@@ -4632,13 +4633,11 @@ func (b *LocalBackend) applyPrefsToHostinfoLocked(hi *tailcfg.Hostinfo, prefs ip
 	hi.AllowsUpdate = envknob.AllowsRemoteUpdate() || prefs.AutoUpdate().Apply.EqualBool(true)
 
 	// count routes without exit node routes
-	routeCount := 0
 	for _, route := range hi.RoutableIPs {
 		if route.Bits() != 0 {
-			routeCount++
+			metricAdvertisedRoutes.Add(1)
 		}
 	}
-	metricAdvertisedRoutes.SetInt(struct{}{}, int64(routeCount))
 
 	var sshHostKeys []string
 	if prefs.RunSSH() && envknob.CanSSHD() {

util/usermetric/usermetric.go

@@ -33,19 +33,14 @@ func NewMultiLabelMap[T comparable](name string, promType, helpText string) *met
 	return m
 }
 
-// NewMap creates and register a new
-// Map variable with the given name and returns it.
+// Publish declares a named exported variable. This should be called from a
+// package's init function when it creates its Vars.
 //
 // Note that usermetric are not protected against duplicate
 // metrics name. It is the caller's responsibility to ensure that
 // the name is unique.
-func NewMap(name string, promType, helpText string) *metrics.MultiLabelMap[struct{}] {
-	m := &metrics.MultiLabelMap[struct{}]{
-		Type: promType,
-		Help: helpText,
-	}
-	vars.Set(name, m)
-	return m
+func Publish(name string, v expvar.Var) {
+	vars.Set(name, v)
 }
 
 // Handler returns a varz.Handler that serves the userfacing expvar contained