Backpressure when queue is full

[mariusandra]

We uncovered a case where a key customer was losing events due to the way this library is configured/built.

With the flush_interval=2 (default is 0.5), flush_at=100 and max_queue_size=1000 (default 10k), their queue got full during peak hours and they started dropping events.

Their recommended solution is very reasonable:

Ideally the library would provide back pressure by blocking/waiting for acknowledgement when flushing a batch of events vs having to configure a max_queue_size.

We should implement some way to block when the queue is full, or some way to let the user know the queue size.