Add metrics interfaces and members, to demo a way to offer metrics wihout tying down to a specific implementation

[jmason]

I've spiked up a quick demo of what I would say is a reasonable way to offer metrics support in Pushy, without tying that down to a specific backend implementation too tightly.

This should allow implementation against:

• Dropwizard Metrics (v3),
• Yammer Metrics (v2),
• Datadog: https://github.com/indeedeng/java-dogstatsd-client
• and statsd using an interface similar to https://github.com/tim-group/java-statsd-client .

Essentially, the API used by Pushy internally is a simplified version of the Metrics API, but should be implementable for statsd and Datadog easily enough too. I'd be curious if there are other metrics systems this approach would pose a problem for....

[jchambers]

@jmason Thanks a million for this!

For the audience at home, this led to a great discussion in #175. We decided to pursue a slightly different approach (see #251), but that approach is very much inspired by this demo.

I'm going to close this, since I think it's served its purpose admirably. Thanks again!

[jmason]

cheers Jon ;)