Now there are only metrics about total queue length and active rpc handler count. But in the RWQueueRpcExecutor, there are different queues and handlers for read/write/scan request. I thought it is necessary to add more metrics for RWQueueRpcExecutor. When use it in production cluster, we can adjust the config of queues and handlers according to the metrics.
Review url: https://reviews.apache.org/r/54072/