Function import_metadata_entries 

async fn import_metadata_entries(
    tx: &mut Transaction<'_, Sqlite>,
    library_id: i64,
    metadata: Option<IndexMap<Fp, Info, FxBuildHasher>>,
) -> (usize, usize, HashSet<Fp>)

Imports all entries from a .metadata.json file into the database.

Returns (books_imported, reading_states_imported, fingerprints_seen). The fingerprint set is passed to import_orphan_reading_states to skip books whose reading state was already written from this file.