Use either REPLICATION_METRICS_10, REPLICATION_METRICS_9_1, or
        REPLICATION_METRICS_9_1 + REPLICATION_METRICS_9_2, depending on the
        postgres version.
        Uses a dictionnary to save the result for each instance

def _get_replication_metrics(self, key, db):
        """ Use either REPLICATION_METRICS_10, REPLICATION_METRICS_9_1, or
        REPLICATION_METRICS_9_1 + REPLICATION_METRICS_9_2, depending on the
        postgres version.
        Uses a dictionnary to save the result for each instance
        """
        metrics = self.replication_metrics.get(key)
        if self._is_10_or_above(key, db) and metrics is None:
            self.replication_metrics[key] = dict(self.REPLICATION_METRICS_10)
            metrics = self.replication_metrics.get(key)
        elif self._is_9_1_or_above(key, db) and metrics is None:
            self.replication_metrics[key] = dict(self.REPLICATION_METRICS_9_1)
            if self._is_9_2_or_above(key, db):
                self.replication_metrics[key].update(self.REPLICATION_METRICS_9_2)
            metrics = self.replication_metrics.get(key)
        return metrics