Add a new measurement.

        The measurement will be processed by the MongoDB observer
        during a heartbeat event.
        Other observers are not yet supported.

        :param metric_name: The name of the metric, e.g. training.loss
        :param value: The measured value
        :param step: The step number (integer), e.g. the iteration number
                    If not specified, an internal counter for each metric
                    is used, incremented by one.

def log_scalar(self, metric_name, value, step=None):
        """
        Add a new measurement.

        The measurement will be processed by the MongoDB observer
        during a heartbeat event.
        Other observers are not yet supported.

        :param metric_name: The name of the metric, e.g. training.loss
        :param value: The measured value
        :param step: The step number (integer), e.g. the iteration number
                    If not specified, an internal counter for each metric
                    is used, incremented by one.
        """
        # Method added in change https://github.com/chovanecm/sacred/issues/4
        # The same as Experiment.log_scalar (if something changes,
        # update the docstring too!)

        return self._metrics.log_scalar_metric(metric_name, value, step)