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_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)
+	}
+
+}