Factory to create a instance associated with this client.

        :type instance_id: str
        :param instance_id: The ID of the instance.

        :type configuration_name: string
        :param configuration_name:
           (Optional) Name of the instance configuration used to set up the
           instance's cluster, in the form:
           ``projects/<project>/instanceConfigs/<config>``.
           **Required** for instances which do not yet exist.

        :type display_name: str
        :param display_name: (Optional) The display name for the instance in
                             the Cloud Console UI. (Must be between 4 and 30
                             characters.) If this value is not set in the
                             constructor, will fall back to the instance ID.

        :type node_count: int
        :param node_count: (Optional) The number of nodes in the instance's
                            cluster; used to set up the instance's cluster.

        :rtype: :class:`~google.cloud.spanner_v1.instance.Instance`
        :returns: an instance owned by this client.

def instance(
        self,
        instance_id,
        configuration_name=None,
        display_name=None,
        node_count=DEFAULT_NODE_COUNT,
    ):
        """Factory to create a instance associated with this client.

        :type instance_id: str
        :param instance_id: The ID of the instance.

        :type configuration_name: string
        :param configuration_name:
           (Optional) Name of the instance configuration used to set up the
           instance's cluster, in the form:
           ``projects/<project>/instanceConfigs/<config>``.
           **Required** for instances which do not yet exist.

        :type display_name: str
        :param display_name: (Optional) The display name for the instance in
                             the Cloud Console UI. (Must be between 4 and 30
                             characters.) If this value is not set in the
                             constructor, will fall back to the instance ID.

        :type node_count: int
        :param node_count: (Optional) The number of nodes in the instance's
                            cluster; used to set up the instance's cluster.

        :rtype: :class:`~google.cloud.spanner_v1.instance.Instance`
        :returns: an instance owned by this client.
        """
        return Instance(instance_id, self, configuration_name, node_count, display_name)