Hi,
You can get these metrics from the Prometheus endpoint (with per-object metrics enable or using :15692/metrcs/per-object endpoint):
$ curl -s localhost:15692/metrics/per-object | grep ^rabbitmq_queue_messages
rabbitmq_queue_messages_ready{vhost="/",queue="stream"} 128452332
rabbitmq_queue_messages_unacked{vhost="/",queue="stream"} 0
rabbitmq_queue_messages{vhost="/",queue="stream"} 128452332
Everything else is up to your monitoring system - query/scrape one of these endpoints, compare with thresholds and send an alert.
Docs about these endpoints:
Best,