Know when your usage export is final

Every page of GET /v1/usage/export now carries finalThrough: no row settled before it can still appear or change, so a range ending at or before it is complete for good. Each row also says final: true once its sandbox or volume is deleted and every charge it will ever have is in the export. A volume's backups bill until their retention ends, so they are not final when the volume is. The CSV gains a last column, final. See export settled usage.