Enable RpcMetrics units to be configurable

One resulting discussion from HADOOP-16266 was that it would be better for the RPC metrics (processing time, queue time) to be in micro- or nanoseconds, since milliseconds does not accurately capture the processing time of many RPC operations. HADOOP-16266 made some small changes in this direction, but to keep the size of the patch down, we did not make it fully configurable. We can complete that work here.