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>

feature/buildfeatures/feature_health_disabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build ts_omit_health
+
+package buildfeatures
+
+// HasHealth is whether the binary was built with support for modular feature "Health checking support".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_health" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasHealth = false

feature/buildfeatures/feature_health_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_health
+
+package buildfeatures
+
+// HasHealth is whether the binary was built with support for modular feature "Health checking support".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_health" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasHealth = true