Time-bucketed metric values for charting.

granularity (hour | day) forces the bucket grain; omitted, it is auto-selected (hourly for <=24h windows, daily otherwise). hour is capped at 7-day windows. compare (previous_period | previous_week) adds the shifted window's series under a compare key, re-based by bucket index onto this window's axis: each point is {date, compare_date, value}, and unmatched points on either side are returned with a null counterpart rather than dropped (see the note above _compare_block).