When application includes a freerelic gem, it should immediately be able to serve /freerelic/ namespaced actions. Those actions should provide simple authorization mechanism and information about slowest queries and actions in acceptable view.
I do not like the idea to set up a standalone application for that purposes, since it's not a ruby way. It requires too much things to do (import application, configure it, set up nginx and deploy it). I believe, it's too complex. Freerelic usage should be simple as 1-2-3.