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