The patch allows alternate HRegionServer subclasses (and in the future coprocessors) to access HBaseRpcMetrics in order to register additional RPC methods into the metrics registry.

The changes are:
 * add getRpcMetrics() to HBaseServer to allow access to the HBaseRpcMetrics instance
 * add getRpcMetrics() to RegionServerServices (and HRegionServer) to allow for future coprocessor accounting
 * add HBaseRpcMetrics.createMetrics(Class[], boolean) overload -- if boolean is true, the registered method names will be prefixed with the class name.  This should help clarify origin for custom metrics and help prevent collisions.

Created new org.apache.hadoop.hbase.regionserver.TestRpcMetrics test case to verify registration and incrementing of metrics from HRegionServer subclasses.