code: func WithValue(key interface{}, val interface{}) func(next http.Handler) http.Handler {
  	return func(next http.Handler) http.Handler {
  		fn := func(w http.ResponseWriter, r *http.Request) {
  			r = r.WithContext(context.WithValue(r.Context(), key, val))
  			next.ServeHTTP(w, r)
  		}
  		return http.HandlerFunc(fn)
  	}
  }
docs: // WithValue is a middleware that sets a given key/value in a context chain.