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>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f