code: def external_metadata(self, datasource_type=None, datasource_id=None):
          
          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)
docs: Gets column info from the source system