@abstractproperty<EOL><INDENT>def metrics_extractors(self):<DEDENT>
raise NotImplementedError<EOL>
Describe how to extract metrics from files written by
        benchmark commands.

        Provides metrics extractors for every categories specified
        in the ``execution_matrix`` member method.

        :return: metrics_extractors instances for each category
        :rtype: ``dict of list of hpcbench.api.MetricsExtractor`` instances.
                if there are dedicated extractors for each category.
                Otherwise a ``list of hpcbench.api.MetricsExtractor``
                instances of there are common extractors for all categories,

        The list structure can be skipped when there is one
        element. For instance if there are dedicated extractors for every
        categories:

        >>> def metrics_extractors(self):
                return dict(
                    foo=foo_stdout_extractor(metrics=['rmax', 'efficiency']),
                    bar=[bar_extractor(), foobar_extractor()]
                )

        If there is only one extractor:

        >>> def metrics_extractors(self):
            return foobar_extractor()