Add instrumentation support

We have been using it in production to track influxes in blocked requests. It was also important when we first started using rack-protection to see if maybe we were blocking a high number of legit users. 

Here is an example graphite graph you can produce:

![198 199 69](https://f.cloud.github.com/assets/245680/947923/72521aea-035d-11e3-966b-451572b97852.png)