Currently, we have an implicit bucketing parameter that controls how we round down the min timestamp for bucket at creation time. This value is derived from the collection's 'granularity' value. The 'granularity' also controls the 'bucketMaxSpanSeconds' value, which we store explicitly in the collection's time-series options.

We want to make the rounding parameter explicit as well, and store it as an optional parameter 'bucketRoundingSeconds'.

For this ticket, we will only store the value for newly-created collections. It will still be derived from 'granularity' and cannot be controlled independently. We will defer downgrade work to future tickets.