Metrics whitelist feature

I have implemented a configuration feature that allows users to specify a whitelist of metrics they with to forward on to cloudwatch. If the metric key does not appear in the whitelist config array, then that metric is skipped from being forwarded to cloudwatch. 

This feature is very useful to us as it allows us to have multiple backends, of which only those metrics we need to raise alarms/perform autoscaling based on a specific limit set of metrics. Of course, this allows us to keep costs down. We have over 100+ additional metrics we want to pass through our statsd instance; the costs of passing these metrics to cloudwatch would very quickly mount up, but we do want them forwarded to our mysql backend.

Anyway, have a look at the pull request and let me know if this is something you are interested in integrating with your project. It would be nice if we didn't have to maintain our own fork for this, and that it was part of the root project.

Cheers!
Jalf