I need to access a Prometheus format output from a monitored host but its port only can be accessed from localhost (127.0.0.1), therefore, Prometheus Special agent launched from the Check_MK server will not work.
Can it be turned somehow in a Linux plugin (deployed in /usr/lib/check_mk_agent/plugins) in order to access that Prometheus output through the Check_MK Linux agent port (6556) as any other plugin?
That I don’t think can be done. Prometheus data is not compatible with what you have in Checkmk. You could remote-push to another prometheus instance that Checkmk prometheus would read, but that would introduce a single-point of failure.
The best would be just to re-configure Prometheus exporters so that you can read whatever you want to read, just from the Checkmk Server.
In general you can use a special agent as a agent plugin. You just need to write a wrapper script with all the necessary options the special agent may need.
There is only a special agent for a Prometheus Instance and not for a Prometheus Exporter available. For the latter one I opened a thread and there is a feature request.