code: func Middleware(h http.Handler, r *Resolver) http.Handler {
  	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
  		ip := r.RealIP(req)
  		h.ServeHTTP(w, req.WithContext(context.WithValue(req.Context(), realIPKey, ip)))
  	})
  }
docs: // Middleware is a simple http.Handler middleware that extracts the RealIP from
  // the request and set it on the request context. Anything downstream can then
  // simply call realip.RealIP to extract the real ip from the request.