Ask HN: Tool or framework to monitor arbitrary services and protocols?

This considers simple UP/DOWN monitoring by external observer. This is not about agents running on servers.

I am looking for:

* ability to monitor arbitrary online services/protocols including custom ones

* in practice, the call itself should be pluggable to cover niche and custom use cases

* open source and self-hosted

* nice to have built-in support for common use cases like HTTP, HTTPS, SSH monitoring

I guess I'm looking for a user interface and scheduling framework of otherwise mostly custom-made calls.