feature/condlite/expvar: add expvar stub package when metrics not needed

Saves ~53 KB from the min build.

Updates #12614

Change-Id: I73f9544a9feea06027c6ebdd222d712ada851299
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/condlite/expvar/omit.go

@@ -0,0 +1,11 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ts_omit_debug && ts_omit_clientmetrics && ts_omit_usermetrics
+
+// excluding the package from builds.
+package expvar
+
+type Int int64
+
+func (*Int) Add(int64) {}