Cloud storage size metrics

[VladLazar]

We should add metrics for the amount of storage used by topics with remote write enabled:

• Current size for data stored in the cloud. This metric should account for compaction and retention.
• Total size which includes data stored both locally and in the cloud. The overlap between the two should be subtracted, leaving in the total number of bytes accessible to clients.

JIRA Link: CORE-1060

[mattschumpert]

Is this a dupe of https://github.com/redpanda-data/core-internal/issues/335 @VladLazar if so please close.

cc @weeco

[jcsp]

This isn't a dupe:

• This ticket is about prometheus metrics on a per-topic basis, for time series monitoring.
• https://github.com/redpanda-data/core-internal/issues/335 is about metering, for data precisely bucketed to time intervals, suitable for billing.
• https://github.com/redpanda-data/redpanda/issues/7920 is about admin API for live data, suitable for GUIs like Console to fetch on demand, and also providing a bit more detail so that GUIs can do things like showing the user what offset range is in local vs. remote storage.