Splitting off of HDFS-14403 to track the first part: introduces more fine-grained measuring of how a call's processing time is split up.
RpcQueueTime may be negative when the response has to be sent later
Cost-Based RPC FairCallQueue
Enable RpcMetrics units to be configurable
Backport [HADOOP-16266] "Add more fine-grained processing time metrics to the RPC layer" to branch-2