Retention policy other than duration

[leeprevost]

Proposal:
Provide retention options based on size or number of records rather than just duration

Current behavior:
Bucket Retention Period

https://docs.influxdata.com/influxdb/v2.5/reference/internals/data-retention/#bucket-retention-period

Desired behavior:

"1000 records"

Alternatives considered:
A carry forward query

Use case:
Some use cases are seasonal meaning there is valid data in last duration (Eg 30days) but not if at beginning of season (ie equipment was turned off over winter) where there is no data in a query last 30 days. For storing statistical summaries of datasets that are for last x available, where x is number of records, a retention policy broader than just duration would be helpful.