Document Metrics & Profiling provided since 2.2

At the moment, it seems that this is the only documentation on the profiler capabilities within the Java SDK (http://blog.couchbase.com/whats-new-couchbase-java-sdk-12). Since it was specific to version 1.2, it might not be obvious to someone looking for how to enable it in the latest. It's also not findable from within our main documentation so that would be helpful as well