add region level metrics readReqeustCount and writeRequestCount

Request rate at region server level can help identify the hot region server. But it will be good if we can further identify the hot regions on that region server. That way, we can easily find out unbalanced regions problem.

Currently, readRequestCount and writeReqeustCount per region is exposed at webUI. It will be more useful to expose it through hadoop metrics framework and/or JMX, so that people can see the history when the region is hot.

I am exposing the existing readRequestCount/writeRequestCount into the dynamic region level metrics framework. I am not changing/exposing it as rate because our openTSDB is taking the raw data of read/write count, and apply rate function to display the rate already.