Information about longest queries and what metrics to use to monitor them

Hi all, just started with MongoDB not so long ago and i was wondering what would be a proper discription of ‘longest queries’ in mongodb and what metrics can i use to monitor them.
currently i am using mongodb_exporter(0.20.9) to exporter mongodb metrics along with prometheus and grafana to moniter my mongodb replica set.

Any help would be appriciated.
Thanks.