Expose retention tuning knobs

[aieri]

Enhancement Proposal

Clouds come in various sizes, and customer requirements can vary wildly. If Loki is to become the default centralized logging solution for all charmed deployments I feel it would be important to allow operators to control their retention period and log storage consumption.

[simskij]

As mentioned in conversations elsewhere, size-based retention is currently not a thing in Loki. That addition to upstream is tracked here.

As for time-based retention, surfacing these would make sense, even thought it would be somewhat of a nightmare for the administrator to calculate with any confidence.