Improve metrics collection and reporting

[skywhale]

[strohel]

Would this mean providing something more than just NoopMetricsHandler, or perhaps even more thorough changes?

I'm on the fence, it could be the case that "easy" use-cases don't care about metrics that much, and "advanced" use-cases may want to use own metrics handler anyway.

Perhaps the goal could be that the "core" is lean, but still able to collect metrics user-supplied code (or some actor-metrics future crate)? Somewhat related to #27 may or may not help with this.

Probably not necessarily Before Release thing, current metrics collection seems to be okayish.

[bschwind]

One crate we were eyeing is metrics, which is a metrics facade similar to log. In general I would like to remove the generic bound on MetricsHandler since it complicates the code a bit.

[mcginty]

Yeah, maybe having an optional metrics feature would be the right way to do this (and when enabled, reports metrics using that metrics crate).