Encountered a scenario where the Reconnection executor becomes backlogged because reconnection attempts to hosts were delayed or timing out.
It would be useful for debugging to have Gauges on all Executor's queue sizes in Cluster so we can detect when they become backlogged.
Adds 4 new metrics:
executor-queue-depth
blocking-executor-queue-depth
reconnection-scheduler-task-count
task-scheduler-task-count
These can be used for monitoring whether or not a Cluster's executors are becoming backlogged which could help understand abnormal behavior of the driver.