Time-bucketed usage per metered feature over a range

Time-bucketed usage per metered feature over a range.

enabled=false mirrors the matrix kill-switch behavior. series is Autumn's per-bin list (each {period, values: {feature_id: n}}); totals is the per-feature {count, sum} over the whole range. period is an epoch timestamp in Autumn's native unit — the frontend renders it as-is.