health: make it omittable

Saves 86 KB.

And stop depending on expvar and usermetrics when disabled,
in prep to removing all the expvar/metrics/tsweb stuff.

Updates #12614

Change-Id: I35d2479ddd1d39b615bab32b1fa940ae8cbf9b11
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

health/usermetrics_omit.go

@@ -0,0 +1,8 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ts_omit_health || ts_omit_usermetrics
+
+package health
+
+func (t *Tracker) SetMetricsRegistry(any) {}