This considers simple UP/DOWN monitoring by external observer. This is <i>not</i> about agents running on servers.<p>I am looking for:<p>* ability to monitor arbitrary online services/protocols including custom ones<p>* in practice, the call itself should be pluggable to cover niche and custom use cases<p>* open source and self-hosted<p>* nice to have built-in support for common use cases like HTTP, HTTPS, SSH monitoring<p>I guess I'm looking for a user interface and scheduling framework of otherwise mostly custom-made calls.