Base metrics

So there are several ways to make sure these go to Librato. I ended up copying this little layer from API because it's by far more lean and abstract than the options:

- Librato has their own gem, but it transmits via API calls as opposed to logs. This makes me nervous because I don't know how they're doing that reliably, without leaking memory, blocking, etc. Also not a fan of having a librato-specific dependency obviously.

- Some gems like [ruby-metrics](https://github.com/johnewart/ruby-metrics) support different backends, but none apparently transmits metrics via logs. This seems optimized for doing some aggregation locally, whereas for our web process we just want to fire and forget.


Not super happy with this but see https://github.com/interagent/pliny-template/pull/141