def METHOD_NAME(self, extra: dict | None = None) -> ExchangeObject:
    """
    Get summary statistics about the local data.
    Args:
        extra: Dict with additional information that can be provided by the FL system.
            For example, requested statistics.
    Returns:
        ExchangeObject: summary statistics.
    Extra dict example::
        requested_stats = {
            FlStatistics.STATISTICS: metrics,
            FlStatistics.NUM_OF_BINS: num_of_bins,
            FlStatistics.BIN_RANGES: bin_ranges
        }
    Returned ExchangeObject example::
        ExchangeObject(
            statistics = {...}
        )
    """
    raise NotImplementedError(f"Subclass {self.__class__.__name__} must implement this method.")