You usually end up with a more imperative part to execute a check or to pull data and push it to a monitoring system. "Call this API / read this file. For each line / json key, mangle into monitoring format. Add in missing metadata. Push all resulting lines to monitoring ingress host".

Beyond that, our icinga has a very declarative config file. "This is host foo.local, it runs a tomcat with application X and a mysql and backups". The system then goes ahead and figures out what kind of checks might be necessary for an application X, or backups.