Instrument thread pool used by Phoenix to help judge pool size and queue depth

To get a better idea of what should be the right thread pool size and the associated queue depth, it is necessary that we instrument the thread pool used by phoenix for servicing requests. An initial list of the metrics that we can capture contains:
1) Time spent by tasks to be picked up by a thread in the pool.
2) Overall time spent by tasks in the pool - from submission to completion
3) Service time - time it took to execute the task
4) Number of requests being submitted to the pool.

It would also be nice to make the choice of using an instrumented thread pool vs a regular one configurable.