code: func httpError(rsp http.ResponseWriter, err error) {
  	rsp.Header().Del(contentEncodingHeader)
  	http.Error(
  		rsp,
  		"An error has occurred while serving metrics:\n\n"+err.Error(),
  		http.StatusInternalServerError,
  	)
  }
docs: // httpError removes any content-encoding header and then calls http.Error with
  // the provided error and http.StatusInternalServerErrer. Error contents is
  // supposed to be uncompressed plain text. However, same as with a plain
  // http.Error, any header settings will be void if the header has already been
  // sent. The error message will still be written to the writer, but it will
  // probably be of limited use.