Billing usage line items add applied_credit object, split by credit type

Billing usage line items now include an applied_credit object. Its type identifies the kind of credit that covers the usage: CREDIT (free credit), PREPAID (credit purchased in advance), or POSTPAID (credit under a paid commitment). Line items for usage that no credit covers omit the object.

Usage covered by different credits now appears as separate line items.

See Get billing usage.