Get current-period usage

What the account has consumed this billing period and what it is estimated to cost. The cost is an ESTIMATE: Stripe rounds each tier up at invoice time, so the settled amount can differ by up to a minute per tier. period_start and period_end are the account's Stripe billing cycle where one resolves, and the calendar month otherwise. GET /v1/usage/instances/{id} reports the same WINDOW, so the two are comparable — but per-instance costs do NOT sum to this total. Each tier is billed on minutes rounded up, and the rounding happens once over the account's aggregate here and once per instance there, so summing the instances overstates the account by up to a minute of each tier's rate per instance. This figure is the account total; use it, not a sum.