Support probe endpoint with domain & host modules

[dragoangel]

I think that usage of /probe endpoint with modules for both domain (mx lookup) and host - can improve a way to interact with exporter in native Prometheus way without requirement to redeploy exporter. It will allow users to dynamically define own targets with each own interval via Probes in k8s.

Thank you for a tool 😊

[uwekamper]

I will have a look if I can add a /probe endpoint to this project.