…uated.

Firstly, we clarify that periods are always whole-minute durations.
Any period that is less than or in between minutes is rounded up to the
next whole-minute.

Secondly, we ensure that the resulting time period is always in the past.
This stops us getting empty metrics for the current-minute period.

Thirdly, we follow the AWS guidelines of aligning periods to whole
multiples within the hour e.g. 10:25->10:30 instead of 10:27->10:32
for a 5 minute period.

…-interval-calculation