presto-mongodb/src/main/java/io/prestosql/plugin/mongodb/MongoSession.java/MongoSession/getColumnMetadata
class MongoSession:    private List<Document> getColumnMetadata(Document doc)
    {
        if (!doc.containsKey(FIELDS_KEY)) {
            return ImmutableList.of();
        }

        return (List<Document>) doc.get(FIELDS_KEY);
    }