Expose exec pool work queue size metrics

[turboFei]

Why are the changes needed?

It can help to know the backend pressure if the exec pool is full.

How was this patch tested?

• Add some test cases that check the changes thoroughly including negative and positive cases if possible

• Add screenshots for manual tests if appropriate

• Run test locally before make a pull request

[bowenliang123]

Good to have the queue size metrics exposed. But where do we have the work queue concept?
As the metrics come from the queue size in execPool, is it good enough to have exec.pool.queue.size as metric name instead?

[turboFei]

I think work queue should be fine, it is a normal concept in ThreadPoolExecutor.

    /**
     * The queue used for holding tasks and handing off to worker
     * threads.  We do not require that workQueue.poll() returning
     * null necessarily means that workQueue.isEmpty(), so rely
     * solely on isEmpty to see if the queue is empty (which we must
     * do for example when deciding whether to transition from
     * SHUTDOWN to TIDYING).  This accommodates special-purpose
     * queues such as DelayQueues for which poll() is allowed to
     * return null even if it may later return non-null when delays
     * expire.
     */
    private final BlockingQueue<Runnable> workQueue;

[bowenliang123]

OK. Just to mention it as we don't have work queue in kyuubi level, and bring it from lower level.

[turboFei]

thanks all, merging to master