Grep metrics by Regexp

Hi!

I want to contribute this very useful feature to the MetricsMvcEndpoint. It allows you to grep metrics by key, so you can query like "/metrics/gauge\.response\.*" to get a list of all response times. 

I should note that this is a breaking change, so please review it and if we need to keep backward compatibility I will revert old implementation and just add another request mapping for it.


Thanks!