Implement occupied_idle metrics for the shipper

[fearful-symmetry]

As part of #11, we need an occupied_idle metric that reports events that have been ack'ed by the output, but are still occupied by the queue. This is part of a series of metrics first proposed by @faec here: elastic/beats#31113 (comment)

Still looking into how to implement this, since it's been months since I've looked at the queue code, but I assume we might need to add something to processACK to track acked events, then subtract that from queue length.

CC @cmacknz

[cmacknz]

Closing, deferred until we have a concrete need for this metric. Let's focus on output metrics.