What should be done?

We should store collections metadata exactly the same way as an old backend does – as *types.Document in a single field, with the same fields, etc. However, the exposed Registry API should be similar to SQLite's backend.

In that issue only the metadata package should be changed, not the new backend package itself.

Where?

https://github.com/FerretDB/FerretDB/tree/main/internal/backends/postgresql/metadata

Definition of Done

• unit tests added;
• spot refactorings done.