"Pain...! Lots of pain..!"
https://www.youtube.com/watch?v=g6e5E20k9jY

as standalone program prometheus is fine (though I'm using #victoriametrics which I whole❤️edly recommend) but actually replacing the whole functionality of nagios+munin will be, erm, adventurous. Using munin is pain-free and automagic, not quite true for prometheus....
Nagios is ugly but the dependency logic works for me very well. I still ought to reconfigure alertmanager, someday.