tsweb: add StatusCodeCounters to HandlerOptions

This lets servers using tsweb register expvars
that will track the number of requests ending
in 200s/300s/400s/500s.

Signed-off-by: Josh Bleecher Snyder <josh@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