Ok, my Raspberry pi VPN router kep crashing (spoiler: it was both overheating and failing to load
iptables rules) so I needed to try to monitor it to diagnose it. Then I went into a system monitoring rabbit hole.
Time series databases such as RRD are probably handy here. RRD looks simple enough. You just run a job that measures something at intervals and it records it for you. The tides of hype have surfaces some more fancy and/or turnkey solutions.
netdata has been recommended to me as something with native raspberry pi support (i.e. it looks for overheating). Open source.
If I wanted to monitor only server traffic logs, I could use goaccess.