Returns the value of Operation.metadata from the last call to
        OperationsClient.get_operation (or if only the initial API call has been
        made, the metadata from that first call).

def metadata(self):
        """Returns the value of Operation.metadata from the last call to
        OperationsClient.get_operation (or if only the initial API call has been
        made, the metadata from that first call).
        """
        # Check exceptional case: return none if no metadata
        if not self._operation.HasField('metadata'):
            return None

        # Return expected metadata
        return _from_any(self._metadata_type, self._operation.metadata)