code: public java.lang.String getResponseBody() {
      java.lang.Object ref = responseBody_;
      if (ref instanceof java.lang.String) {
        return (java.lang.String) ref;
      } else {
        com.google.protobuf.ByteString bs = 
            (com.google.protobuf.ByteString) ref;
        java.lang.String s = bs.toStringUtf8();
        responseBody_ = s;
        return s;
      }
    }
docs: <pre>
  Optional. The name of the response field whose value is mapped to the HTTP
  response body. When omitted, the entire response message will be used
  as the HTTP response body.
  NOTE: The referred field must be present at the top-level of the response
  message type.
  </pre>
  
  <code>string response_body = 12;</code>