contrib/gorilla/mux: add router wrapper

[Julio-Guerra]

Based on user feedback we had that didn't want to totally replace their gorilla/mux imports by ours, we would like to propose them the alternative of rather importing our contrib to wrap their gorilla router instead. To do so, we introduced a new function called WrapRouter() that wraps the given gorilla mux router with our wrapper.

Note that a pure gorilla middleware is not a good option for us because they do not get called when the route is not found, leading the missing all the 404 on routes that were not found on the router. This impacts both the APM tracing and the AppSec monitoring. So the current solution with WrapRouter instead is the best possible solution to be able to monitor every request entering the router.