package main

func main() {
  r := mux.NewRouter()
  mount(r, "/api/v1", handler.Router(), auth)
  ...
}

// mount will take routers from a handler and add it with a pathprefix
// mount will take routers from a handler and add it with a pathprefix
func mount(r *mux.Router, path string, handler http.Handler, middleware mux.MiddlewareFunc) {
	r.PathPrefix(path).Handler(
		http.StripPrefix(
			strings.TrimSuffix(path, "/"),
			middleware(handler),
		),
	)
}