code: public java.lang.String getServingData() {
      java.lang.Object ref = servingData_;
      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();
        servingData_ = s;
        return s;
      }
    }
docs: <pre>
  Any data that was used to serve this request. For example, an encrypted
  stack trace that can be sent back to the service provider for debugging.
  </pre>
  
  <code>string serving_data = 2;</code>