usermetric: add initial user-facing metrics

This commit adds a new usermetric package and wires
up metrics across the tailscale client.

Updates tailscale/corp#22075

Co-authored-by: Anton Tolchanov <anton@tailscale.com>
Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

ipn/ipnlocal/local.go

@@ -106,6 +106,7 @@ import (
 	"tailscale.com/util/systemd"
 	"tailscale.com/util/testenv"
 	"tailscale.com/util/uniq"
+	"tailscale.com/util/usermetric"
 	"tailscale.com/version"
 	"tailscale.com/version/distro"
 	"tailscale.com/wgengine"
@@ -117,6 +118,9 @@ 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 {
@@ -4646,6 +4650,15 @@ func (b *LocalBackend) applyPrefsToHostinfoLocked(hi *tailcfg.Hostinfo, prefs ip
 	hi.ShieldsUp = prefs.ShieldsUp()
 	hi.AllowsUpdate = envknob.AllowsRemoteUpdate() || prefs.AutoUpdate().Apply.EqualBool(true)
 
+	// count routes without exit node routes
+	var routes int64
+	for _, route := range hi.RoutableIPs {
+		if route.Bits() != 0 {
+			routes++
+		}
+	}
+	metricAdvertisedRoutes.Set(float64(routes))
+
 	var sshHostKeys []string
 	if prefs.RunSSH() && envknob.CanSSHD() {
 		// TODO(bradfitz): this is called with b.mu held. Not ideal.