As we look to add github.com/prometheus/client_golang/prometheus to
more parts of the codebase, lock in that we don't use it in tailscaled,
primarily for binary size reasons.
Updates #12614
Change-Id: I03c100d12a05019a22bdc23ce5c4df63d5a03ec6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>