expose tailscaled_advertised_routes as a gauge

ipn/ipnlocal/local.go

@@ -12,7 +12,6 @@
 	"encoding/base64"
 	"encoding/json"
 	"errors"
-	"expvar"
 	"fmt"
 	"io"
 	"log"
@@ -119,11 +118,8 @@
 	"tailscale.com/wgengine/wgcfg/nmcfg"
 )
 
-var metricAdvertisedRoutes expvar.Int
-
-func init() {
-	usermetric.Publish("tailscaled_advertised_routes", &metricAdvertisedRoutes)
-}
+var metricAdvertisedRoutes = usermetric.NewGauge(
+	"tailscaled_advertised_routes", "Number of routes advertised by tailscaled")
 
 var controlDebugFlags = getControlDebugFlags()
 
@@ -4633,11 +4629,13 @@ func (b *LocalBackend) applyPrefsToHostinfoLocked(hi *tailcfg.Hostinfo, prefs ip
 	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 {
-			metricAdvertisedRoutes.Add(1)
+			routes++
 		}
 	}
+	metricAdvertisedRoutes.Set(float64(routes))
 
 	var sshHostKeys []string
 	if prefs.RunSSH() && envknob.CanSSHD() {

util/usermetric/usermetric.go

@@ -7,6 +7,8 @@
 
 import (
 	"expvar"
+	"fmt"
+	"io"
 	"net/http"
 
 	"tailscale.com/metrics"
@@ -33,14 +35,40 @@ func NewMultiLabelMap[T comparable](name string, promType, helpText string) *met
 	return m
 }
 
-// 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 Publish(name string, v expvar.Var) {
-	vars.Set(name, v)
+// Gauge is a gauge metric with no labels.
+type Gauge struct {
+	m    *expvar.Float
+	help string
+}
+
+// NewGauge creates and register a new gauge metric with the given name and help text.
+func NewGauge(name, help string) *Gauge {
+	m := &expvar.Float{}
+	vars.Set(name, m)
+	return &Gauge{m, help}
+}
+
+// Set sets the gauge to the given value.
+func (g *Gauge) Set(v float64) {
+	g.m.Set(v)
+}
+
+// WritePrometheus writes the gauge metric in Prometheus format to the given writer.
+// This satisfies the varz.PrometheusWriter interface.
+func (g *Gauge) WritePrometheus(w io.Writer, name string) {
+	io.WriteString(w, "# TYPE ")
+	io.WriteString(w, name)
+	io.WriteString(w, " gauge\n")
+	if g.help != "" {
+		io.WriteString(w, "# HELP ")
+		io.WriteString(w, name)
+		io.WriteString(w, " ")
+		io.WriteString(w, g.help)
+		io.WriteString(w, "\n")
+	}
+
+	io.WriteString(w, name)
+	fmt.Fprintf(w, " %v\n", g.m.Value())
 }
 
 // Handler returns a varz.Handler that serves the userfacing expvar contained

util/usermetric/usermetric_test.go

@@ -0,0 +1,25 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package usermetric
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestGauge(t *testing.T) {
+	g := NewGauge("test_gauge", "This is a test gauge")
+	g.Set(15)
+
+	var buf bytes.Buffer
+	g.WritePrometheus(&buf, "test_gauge")
+	const want = `# TYPE test_gauge gauge
+# HELP test_gauge This is a test gauge
+test_gauge 15
+`
+	if got := buf.String(); got != want {
+		t.Errorf("got %q; want %q", got, want)
+	}
+
+}