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>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f