Factory:  construct an instance from a protobuf.

        :type operation_pb:
            :class:`~google.longrunning.operations_pb2.Operation`
        :param operation_pb: Protobuf to be parsed.

        :type client: object: must provide ``_operations_stub`` accessor.
        :param client: The client used to poll for the status of the operation.

        :type caller_metadata: dict
        :param caller_metadata: caller-assigned metadata about the operation

        :rtype: :class:`Operation`
        :returns: new instance, with attributes based on the protobuf.

def from_pb(cls, operation_pb, client, **caller_metadata):
        """Factory:  construct an instance from a protobuf.

        :type operation_pb:
            :class:`~google.longrunning.operations_pb2.Operation`
        :param operation_pb: Protobuf to be parsed.

        :type client: object: must provide ``_operations_stub`` accessor.
        :param client: The client used to poll for the status of the operation.

        :type caller_metadata: dict
        :param caller_metadata: caller-assigned metadata about the operation

        :rtype: :class:`Operation`
        :returns: new instance, with attributes based on the protobuf.
        """
        result = cls(operation_pb.name, client, **caller_metadata)
        result._update_state(operation_pb)
        result._from_grpc = True
        return result