Returns a Source that can be fed via the materialized queue.

The queue length counter can at most be eventually consistent due to the counter increment and decrement operation being scheduled separately and possibly not in the same order as the actual enqueuing and dequeueing of items.

For this reason, you may also read values on the saturation counter which are negative or exceed bufferSize.

Note that the fact that the count is decremented in a second operator means that its buffering will likely skew the measurements to be greater than the actual value, rather than the other way around.

We track the queue capacity as a counter as we may want to aggregate the metrics for multiple individual queues of the same kind and we want to be able to decrease the capacity when the queue gets completed.