Capture reftable information in repository stats

From @pks-gitlab comment here

I think it would be interesting regardless to surface information specific to the reftable backend:

• The length of tables.list.
• A list of tables, each with their minimum/maximum update index and file size. This shouldn't be too large given that the number of reftables is essentially bounded.
• The number of unrecognized garbage files.

The reason is that the repository info isn't only used as a mechanism for housekeeping, but is also being logged. As we have a rather high likelihood of reftable-specific bugs due to it being so new this information, I think that this information would be really helpful to figure out whether it's working as intended.

We can also defer this to a follow-up merge request, but in that case we should adapt the comment and create and schedule an issue.