tsweb, derp: add expvar http.Handler for Prometheus's format

And add some opinions.

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

metrics/metrics.go

@@ -0,0 +1,23 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package metrics contains expvar & Prometheus types and code used by
+// Tailscale for monitoring.
+package metrics
+
+import "expvar"
+
+// Map is a string-to-Var map variable that satisfies the expvar.Var
+// interface.
+//
+// Semantically, this is mapped by tsweb's Prometheus exporter as a
+// collection of unrelated variables exported with a common prefix.
+//
+// This lets us have tsweb recognize *expvar.Map for different
+// purposes in the future. (Or perhaps all uses of expvar.Map will
+// require explicit types like this one, declaring how we want tsweb
+// to export it to Prometheus.)
+type Set struct {
+	expvar.Map
+}