Support custom sinks

[jorendorff]

Add a method to the builder that allows users to specify their own MetricSink instead of unconditionally using UDP.

This is an alternate way to do what #22 is doing, and make the functionality a little more discoverable maybe. It's honestly not a ton better. Awkward -- the "builder pattern" is clunky for what we're doing.

Includes docs and tests, though.

Edit: Let's not land this until we decide what to do in #22. It would be awfully nice to support "don't lose metrics on shutdown" out of the box.

[mbellani]

It would be awfully nice to support "don't lose metrics on shutdown" out of the box.

Linked cadence issue from #22 mentions that they want to flush metrics during a quiet period. I guess the goal with such a feature would be that you get the metrics reported consistently vs relying on shutdown when your buffer is not full. I do think that we had seen a drop method implemented somewhere in cadence that does flush before shutting down, so I'm not sure if more is needed to be done but we always validate that it's doing what 'it's supposed to.

[jorendorff]

I do think that we had seen a drop method implemented somewhere in cadence that does flush before shutting down, so I'm not sure if more is needed to be done but we always validate that it's doing what 'it's supposed to.

In cadence, yes. But I don't think we tell metrics to drop the recorder. In fact, I think the only way metrics offers to do that is this hidden, unsafe function. So AFAIK, cadence's destructor is not called in our case.

Destructors are not called for global static variables in Rust:

Static items do not call drop at the end of the program.

[GeorgeHahn]

Silently overriding the other sink configs is a pretty big surprise. Maybe this could be implemented here in a way that bypasses the builder type?

For example: fn StatsdBuilder::from_sink<T>(sink: T) -> StatsdRecorder

That being said, I'm not sad about the public StatsdRecorder usage either:

let sink = build_sink();
let statsd_client = StatsdClient::from_sink("prefix", sink);
let recorder = StatsdRecorder::new(statsd_client, HistogramType::Histogram);
metrics::set_boxed_recorder(Box::new(recorder))
  .expect("metrics recorder should be uninitialized");

[jorendorff]

I feel a little sheepish about this, but I sketched it out in the way you suggest, and I think this is probably the least awkward way. In any case we can improve on it later. Thanks for your patience.

[mbellani]