If you can use GenRmq then it has message count, message_count functions.

@collegeimprovements Oh thanks - that led me to the underlying function: AMQP.Queue — amqp v1.4.2 . Works great!

For anyone who stumbles on this later: AMQP only supports getting the number of messages that are “Ready”, not those that are “unacked”. For the latter, you might have to use RabbitMQ’s REST API. See https://stackoverflow.com/questions/63018151/can-i-see-the-count-of-unacked-messages-in-rabbitmq-via-an-amqp-client

My system that consumes from the queue is using Broadway. In normal load, I basically never see any messages as “ready” because they’re quickly grabbed up by my Broadway pipeline. By queueing a lot of messages and putting a Process.sleep(:infinity) in my handle_message, I can see that my system will have at most N unacked messages at a time. So if I see any messages in the “ready” count, I can basically assume that there are N more being handled at the moment, which is a good enough indicator to decide whether the system is getting overloaded.