I've read that it's possible to monitor Wildfly with Nagios in links like this one and I also know that there are solutions that provide that service as well.
Does anybody knows how to do that, how to monitor Wildfly with Nagios, any recommendations on how to start? Any reference would be very appreciated.
You can consider using JSON based HTTP management API. Sample plugin (Python based) for JBossAS - standalone mode is available here https://github.com/aparnachaudhary/nagios-plugin-jbossas7. This should also work for WildFly.
Some details about WildFly HTTP Management API can be found here https://docs.jboss.org/author/display/WFLY9/The+HTTP+management+API