Server should not be registering response code metrics

[jkinkead]

The Server class is registering response code metrics in its constructor, via a call to a helper addResponseCodeMeters.

This is a weird place for this to happen, and it's totally different from where the other per-request metrics are created and tracked (in the CompiledRoutes constructor).

We should pick a place this happens, and handle it in an organized way.

Our rate limits, firewall, and IP white/black lists also track metrics, but do so in a localized way which seems fine.

[jkinkead]

It would also be nice if the response code meters worked for ALL response codes, not just those that were pre-registered. Right now, any responses outside of the known codes are silently dropped from counting.