Add #count and #measure methods to Pliny

[gudmundur]

Adds Pliny::Metrics.count(*names, value: 1) and Pliny::Metrics.measure(*names, &block) methods. By invoking the methods, they will emit logfmt entries.

Examples

• Pliny::Metrics.count(:hello) will emit count#my_pliny_app.hello=1.
• Pliny::Metrics.measure(:hello) { sleep(1) } will emit measure#my_pliny_app.hello=1.000.

[brandur]

Good stuff! Two thoughts:

1. It may be worth thinking about making the metrics module configurable so that there could be more future options beyond just emitting log lines. I think most of the Heroku/Pliny practices are reasonable enough to emulate elsewhere, but I've always thought of the log-based emission somewhat contentious just because it's quite inefficient/expensive compared to say a UDP-based solution for example.
2. Should we force the metrics helpers to be referenced at Pliny::Metrics rather than mixing them into the top level namespace? It's a little more typing, but I don't think the explicitness is a bad thing.

[gudmundur]

@brandur I've moved the methods into Pliny::Metrics. As for abstracting away how the metrics are emitted, I'd prefer to punt on that until that use case actually comes up. Is that alright with you?

[brandur]

@brandur I've moved the methods into Pliny::Metrics. As for abstracting away how the metrics are emitted, I'd prefer to punt on that until that use case actually comes up. Is that alright with you?

@gudmundur Yep, absolutely! It seems like the interface should be generic enough that it won't back us into a corner.

LGTM! Sorry about the review delay. Feel free to bump these more often if I'm not answering because it might mean that I declared inbox zero on GitHub issues :)