Gets column info from the source system

def external_metadata(self, datasource_type=None, datasource_id=None):
        """Gets column info from the source system"""
        if datasource_type == 'druid':
            datasource = ConnectorRegistry.get_datasource(
                datasource_type, datasource_id, db.session)
        elif datasource_type == 'table':
            database = (
                db.session
                .query(Database)
                .filter_by(id=request.args.get('db_id'))
                .one()
            )
            Table = ConnectorRegistry.sources['table']
            datasource = Table(
                database=database,
                table_name=request.args.get('table_name'),
                schema=request.args.get('schema') or None,
            )
        external_metadata = datasource.external_metadata()
        return self.json_response(external_metadata)