Hi and thanks for your question!
Maybe what's happening here is that JBoss creates a special MBeanServer where it registers the metrics to. Currently, we only track metrics from the platfrom MBean server (ManagementFactory.getPlatformMBeanServer()).
Could you try to find out if there are indeed multiple MBean servers by calling MBeanServerFactory.findMBeanServer(null)? If so, whats the domain (MBeanServer#getDefaultDomain) of the MBean server that contains the metrics you are looking for?