HBASE-3405: Allow HBaseRpcMetrics to register custom interface methods

Review Request #1414 - Created Jan. 5, 2011 and updated

Gary Helmling
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.