Allow selection of which metrics to submit from Librato metrics reporter

[ryanlower]

Can specific :only or :except when initializing the reporter, e.g.

Metriks::Reporter::LibratoMetrics.new(
  'email',
  'key',
  {
    prefix: 'example',
    only: [
      :count,
      :mean,
      :one_minute_rate
    ]
  }
).start

[eric]

This is great. It may be a couple days before I get a chance to merge and release this — I'm traveling in Japan at the moment... but this looks useful.

Thanks for the pull request.

[eric]

Defaulting these to an empty array is going to cause the if's later on to always evaluate to true.

I think this may be worth a test case to not break things.

[douglasdollars]

This one is going to really come in handy. Thanks guys!

[eric]

With the defaulting above, this is going to need to be if @only && !@only.empty? — same with the @except one.