[logback-user] How can I monitor the actual size of the asyncappender queue?

I believe I now understand the basic mechanics of the AsyncAppender queue, and the tradeoffs of having it small or large. What's not obvious to me is how I can monitor the utilization of the queue, perhaps in a load test. I did notice there is a "listener" mechanism, and one example I believe illustrates how I can detect when logs are dropped because the queue is full, but I want to get an idea of how well the queue size is being utilized, or how close it's getting to being full.