rack/attack: collect metrics on rate limiting
rack/attack: collect metrics on rate limiting

This patch introduces some instrumentation for rack/attack that collects
three types of metrics:

* number of requests blocked by matched rule
* blocked requests by ip
* blocked requests by api endpoint

The metrics are reported to librato via metriks. The instrumentation
can be enabled and disabled via the RACK_ATTACK_INSTRUMENT env var.

travis-ci/travis-api,travis-ci/travis-api,travis-ci/travis-api