Billing based on the last value sent by the user (for usage-based charges).

Proposed solution: new aggregation type, similar to Stripe's last_ever option (see description below).

Extract from the Stripe documentation:

"last_ever – The total billed is based on the most recently provided usage record. If no usage is reported during the current billing period, Stripe looks for a previous usage record. If no usage record is found, the total billed is based on a usage quantity of 0."