munin is the veteran of monitoring. It offers an easy plugin API, graphs and alerts. I use it for everything (temperarure, stock prices, API of the local water park etc.) since with curl, jq + bash you can create a plugin in minutes (or any other language you like).

There are much more modern solutions, just putting it here in case you want some low maintenance, tried & proven, rock solid solution that “just works”. It’s written in perl and maintained by a nice community.