Some people are probably going to throw some shade on me for saying this since it's so out of fashion but in my mind, when it comes to some types of basic monitoring (SNMP monitoring of switches/linux servers, disk space usage, backups running and handling them when they don't) then Nagios does get the job done. It's definitely olives and not candy[1] but it's stable, modular, relatively easy to configure (when you g…
It then rots within your infrastructure, because it resists being configured any way _except_ by hand. I've built two systems for configuration-management of Nagios (at different companies), and it's an unpleasant problem to solve.
Prometheus's metric format and query syntax are cool, but the real star of the design is simply this: you don't have to restart it, or even change files on your Prometheus server, when you add or remove servers from your environment.