Retrieve monthly stats for all subusers

**This endpoint allows you to retrieve the monthly email statistics for all subusers over the given date range.** When using the `sort_by_metric` to sort your stats by a specific metric, you can not sort by the following metrics: `bounce_drops`, `deferred`, `invalid_emails`, `processed`, `spam_report_drops`, `spam_reports`, or `unsubscribe_drops`.