Backpressure / blocking /queue full indication on publisher / subscriber

Hi, what's the recommended way of detecting that a blocking (WAIT_FOR_CONSUMER/BLOCK_PRODUCER) publisher/subscriber pair is subject for backpressure / blocking (at full subscription queue). Would be nice to have a helper function for it and not have to resort to application layer stopwatch timers

Best,

Start by reviewing the publisher/subscriber blocking modes WAIT_FOR_CONSUMER and BLOCK_PRODUCER, focusing on how a full subscription queue is represented and exposed. Compare existing publisher and subscriber APIs and tests, then define the helper's expected indication and testable behavior for a queue under backpressure.

Written by the indexing model from the issue text.