I have recent experience only with: graphite, uptimekuma, moira - I would certainly recommend for small shops.

I believe what makes it work is the glue, ie. what it integrates with: connect with application logging (log4j), notification with slack.

A few years back I was working with splunk (but this is another galaxy when it comes to cost) and ELK stack. But this was only for logging, not full observability.