Some additional information regarding the wildfly integration and the rest interface in particular:
Any subsystem in wildfly can register an undertow HTTP handler or an undertow HTTP upgrade handler. The HTTP handler exposes the regular undertow API for building HTTP based endpoints [1]. This one should be straight forward to use for implementing the REST interface you currently have in rhq-metrics.
The upgrade handler on the other hand does HTTP upgrade to a custom protocol of your choice (if needed in the future). It can as well be used to provide web socket based access to your services.
Taking into consideration that undertow can also be easily embedded [2] I think this covers a lot of ground.
Regards, Heiko