RFC: Tombstones based deletes in Cortex

[sandeepsukhani]

As of now, there is no way of deleting unwanted or sensitive data from Cortex. Here is the design doc for implementing deletes in Cortex using Tombstones. It would be great if someone can review it and help it align with the expectations of the users.

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 30 days. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.

[aengusrooneygrafana]

Updating with the latest design doc (reference above is out of date - the doc now marked as 'old') - the Cortex team are working on the latest design doc here

[bboreham]

Implemented in #2103 and subsequent PRs.
Docs https://cortexmetrics.io/docs/guides/deleting-series/