tsweb: cache prometheus metric names & types

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

tsweb/tsweb.go

@@ -455,18 +455,30 @@ func WritePrometheusExpvar(w io.Writer, kv expvar.KeyValue) {
 	writePromExpVar(w, "", kv)
 }
 
-func writePromExpVar(w io.Writer, prefix string, kv expvar.KeyValue) {
-	key := kv.Key
+type prometheusMetricDetails struct {
+	Name  string
+	Type  string
+	Label string
+}
+
+var prometheusMetricCache sync.Map // string => *prometheusMetricDetails
+
+func prometheusMetric(prefix string, key string) (string, string, string) {
+	cachekey := prefix + key
+	if v, ok := prometheusMetricCache.Load(cachekey); ok {
+		d := v.(*prometheusMetricDetails)
+		return d.Name, d.Type, d.Label
+	}
 	var typ string
 	var label string
 	switch {
-	case strings.HasPrefix(kv.Key, gaugePrefix):
+	case strings.HasPrefix(key, gaugePrefix):
 		typ = "gauge"
-		key = strings.TrimPrefix(kv.Key, gaugePrefix)
+		key = strings.TrimPrefix(key, gaugePrefix)
 
-	case strings.HasPrefix(kv.Key, counterPrefix):
+	case strings.HasPrefix(key, counterPrefix):
 		typ = "counter"
-		key = strings.TrimPrefix(kv.Key, counterPrefix)
+		key = strings.TrimPrefix(key, counterPrefix)
 	}
 	if strings.HasPrefix(key, labelMapPrefix) {
 		key = strings.TrimPrefix(key, labelMapPrefix)
@@ -474,7 +486,18 @@ func writePromExpVar(w io.Writer, prefix string, kv expvar.KeyValue) {
 			label, key = a, b
 		}
 	}
-	name := strings.ReplaceAll(prefix+key, "-", "_")
+	d := &prometheusMetricDetails{
+		Name:  strings.ReplaceAll(prefix+key, "-", "_"),
+		Type:  typ,
+		Label: label,
+	}
+	prometheusMetricCache.Store(cachekey, d)
+	return d.Name, d.Type, d.Label
+}
+
+func writePromExpVar(w io.Writer, prefix string, kv expvar.KeyValue) {
+	key := kv.Key
+	name, typ, label := prometheusMetric(prefix, key)
 
 	switch v := kv.Value.(type) {
 	case PrometheusVar:

tsweb/tsweb_test.go

@@ -450,15 +450,15 @@ func TestVarzHandler(t *testing.T) {
 		},
 		{
 			"func_float64_gauge",
-			"gauge_x",
+			"gauge_y",
 			expvar.Func(func() any { return float64(1.2) }),
-			"# TYPE x gauge\nx 1.2\n",
+			"# TYPE y gauge\ny 1.2\n",
 		},
 		{
 			"func_float64_untyped",
-			"x",
+			"z",
 			expvar.Func(func() any { return float64(1.2) }),
-			"x 1.2\n",
+			"z 1.2\n",
 		},
 		{
 			"metrics_label_map",