docs: def download_metadata_tar(self,download_dir):  
          """
          Downloads the ${run_name}.metadata.tar file from the DNAnexus sequencing results project.
      
          Args: 
              download_dir: `str` - The local directory path to download the QC report to.
          Returns:
              `str`: The filepath to the downloaded metadata tarball.
          """
          if not os.path.isdir(download_dir):
              os.makedirs(download_dir)  
          res = dxpy.find_one_data_object(project=self.dx_project_id,folder=self.DX_RAW_DATA_FOLDER,name="*metadata.tar",name_mode="glob")
          #res will be something like {u'project': u'project-BzqVkxj08kVZbPXk54X0P2JY', u'id': u'file-BzqVkg800Fb0z4437GXJfGY6'}
          #dxpy.find_one_data_object() raises a dxpy.exceptions.DXSearchError() if nothing is found.
          dx_file = dxpy.DXFile(dxid=res["id"],project=res["project"])
          download_file_name = os.path.join(download_dir,dx_file.name)
          msg = "{filename} to {download_dir}.".format(filename=dx_file.name,download_dir=download_dir)
          debug_logger.debug("Downloading " + msg)
          dxpy.bindings.dxfile_functions.download_dxfile(dxid=dx_file,filename=download_file_name)
          success_logger.info("Downloaded " + msg)
          return download_file_name