code: def get_measurement_metadata(self, fields, ids=None, noneval=nan,
                                   output_format='DataFrame'):
          
          fields = to_list(fields)
          func = lambda x: x.get_meta_fields(fields)
          meta_d = self.apply(func, ids=ids, applyto='measurement',
                              noneval=noneval, output_format='dict')
          if output_format is 'dict':
              return meta_d
          elif output_format is 'DataFrame':
              from pandas import DataFrame as DF
              meta_df = DF(meta_d, index=fields)
              return meta_df
          else:
              msg = ("The output_format must be either 'dict' or 'DataFrame'. " +
                     "Encountered unsupported value %s." % repr(output_format))
              raise Exception(msg)
docs: Get the metadata fields of specified measurements (all if None given).
  
          Parameters
          ----------
          fields : str | iterable of str
              Names of metadata fields to be returned.
          ids : hashable| iterable of hashables | None
              Keys of measurements for which metadata will be returned.
              If None is given return metadata of all measurements.
          noneval : obj
              Value returned if applyto is 'data' but no data is available.
          output_format :  'DataFrame' | 'dict'
              'DataFrame' : return DataFrame,
              'dict'      : return dictionary.
  
          Returns
          -------
          Measurement metadata in specified output_format.