metrics, tsweb/varz: add multi-label map metrics

Updates tailscale/corp#18640

Change-Id: Ia9ae25956038e9d3266ea165537ac6f02485b74c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

metrics/multilabelmap_test.go

@@ -0,0 +1,121 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package metrics
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"testing"
+)
+
+type L2 struct {
+	Foo string `prom:"foo"`
+	Bar string `prom:"bar"`
+}
+
+func TestMultilabelMap(t *testing.T) {
+	m := new(MultiLabelMap[L2])
+	m.Add(L2{"a", "b"}, 2)
+	m.Add(L2{"b", "c"}, 4)
+	m.Add(L2{"b", "b"}, 3)
+	m.Add(L2{"a", "a"}, 1)
+
+	cur := func() string {
+		var buf bytes.Buffer
+		m.Do(func(kv KeyValue[L2]) {
+			if buf.Len() > 0 {
+				buf.WriteString(",")
+			}
+			fmt.Fprintf(&buf, "%s/%s=%v", kv.Key.Foo, kv.Key.Bar, kv.Value)
+		})
+		return buf.String()
+	}
+
+	if g, w := cur(), "a/a=1,a/b=2,b/b=3,b/c=4"; g != w {
+		t.Errorf("got %q; want %q", g, w)
+	}
+
+	var buf bytes.Buffer
+	m.WritePrometheus(&buf, "metricname")
+	const want = `metricname{foo="a",bar="a"} 1
+metricname{foo="a",bar="b"} 2
+metricname{foo="b",bar="b"} 3
+metricname{foo="b",bar="c"} 4
+`
+	if got := buf.String(); got != want {
+		t.Errorf("promtheus output = %q; want %q", got, want)
+	}
+
+	m.Delete(L2{"b", "b"})
+
+	if g, w := cur(), "a/a=1,a/b=2,b/c=4"; g != w {
+		t.Errorf("got %q; want %q", g, w)
+	}
+
+	allocs := testing.AllocsPerRun(1000, func() {
+		m.Add(L2{"a", "a"}, 1)
+	})
+	if allocs > 0 {
+		t.Errorf("allocs = %v; want 0", allocs)
+	}
+	m.Init()
+	if g, w := cur(), ""; g != w {
+		t.Errorf("got %q; want %q", g, w)
+	}
+
+	writeAllocs := testing.AllocsPerRun(1000, func() {
+		m.WritePrometheus(io.Discard, "test")
+	})
+	if writeAllocs > 0 {
+		t.Errorf("writeAllocs = %v; want 0", writeAllocs)
+	}
+}
+
+func TestMultiLabelMapTypes(t *testing.T) {
+	type LabelTypes struct {
+		S string
+		B bool
+		I int
+		U uint
+	}
+
+	m := new(MultiLabelMap[LabelTypes])
+	m.Type = "counter"
+	m.Help = "some good stuff"
+	m.Add(LabelTypes{"a", true, -1, 2}, 3)
+	var buf bytes.Buffer
+	m.WritePrometheus(&buf, "metricname")
+	const want = `# TYPE metricname counter
+# HELP metricname some good stuff
+metricname{s="a",b="true",i="-1",u="2"} 3
+`
+	if got := buf.String(); got != want {
+		t.Errorf("got %q; want %q", got, want)
+	}
+
+	writeAllocs := testing.AllocsPerRun(1000, func() {
+		m.WritePrometheus(io.Discard, "test")
+	})
+	if writeAllocs > 0 {
+		t.Errorf("writeAllocs = %v; want 0", writeAllocs)
+	}
+}
+
+func BenchmarkMultiLabelWriteAllocs(b *testing.B) {
+	b.ReportAllocs()
+
+	m := new(MultiLabelMap[L2])
+	m.Add(L2{"a", "b"}, 2)
+	m.Add(L2{"b", "c"}, 4)
+	m.Add(L2{"b", "b"}, 3)
+	m.Add(L2{"a", "a"}, 1)
+
+	var w io.Writer = io.Discard
+
+	b.ResetTimer()
+	for range b.N {
+		m.WritePrometheus(w, "test")
+	}
+}