To drop all metrics except one specific one, use relabeling with keep and a chained regex. E.g. To keep only the RabbitMQ metrics of the async.operations.all queue in all deployment-XXXX namespaces when using a Kubernetes PodMonitoring CRD:

metricRelabeling:

- action: keep
  regex: (deployment-(\d+);rabbitmq_detailed_queue_messages;async\.operations\.all)
  sourceLabels:
    - namespace
    - __name__
    - queue

I recommend this blog post: https://heiioncall.com/guides/the-art-of-metric-relabeling-in-prometheus