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>

util/usermetric/usermetric.go

@@ -0,0 +1,84 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package usermetric provides a container and handler
+// for user-facing metrics.
+package usermetric
+
+import (
+	"expvar"
+	"fmt"
+	"io"
+	"net/http"
+
+	"tailscale.com/metrics"
+	"tailscale.com/tsweb/varz"
+)
+
+var vars expvar.Map
+
+// NewMultiLabelMap creates and register a new
+// MultiLabelMap[T] variable with the given name and returns it.
+// The variable is registered with the userfacing metrics package.
+//
+// Note that usermetric are not protected against duplicate
+// metrics name. It is the caller's responsibility to ensure that
+// the name is unique.
+func NewMultiLabelMap[T comparable](name string, promType, helpText string) *metrics.MultiLabelMap[T] {
+	m := &metrics.MultiLabelMap[T]{
+		Type: promType,
+		Help: helpText,
+	}
+	var zero T
+	_ = metrics.LabelString(zero) // panic early if T is invalid
+	vars.Set(name, m)
+	return m
+}
+
+// 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 {
+	g := &Gauge{&expvar.Float{}, help}
+	vars.Set(name, g)
+	return g
+}
+
+// Set sets the gauge to the given value.
+func (g *Gauge) Set(v float64) {
+	g.m.Set(v)
+}
+
+// String returns the string of the underlying expvar.Float.
+// This satisfies the expvar.Var interface.
+func (g *Gauge) String() string {
+	return g.m.String()
+}
+
+// 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
+// in this package.
+func Handler(w http.ResponseWriter, r *http.Request) {
+	varz.ExpvarDoHandler(vars.Do)(w, r)
+}

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)
+	}
+
+}