rename usermetrics -> usermetric

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

util/usermetric/usermetric.go

@@ -0,0 +1,40 @@
+// 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"
+	"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
+}
+
+// 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)
+}