Use COMMON_ARCHIVER_METRICS to read from pg_stat_archiver as
        defined in 9.4 (first version to have this table).
        Uses a dictionary to save the result for each instance

def _get_archiver_metrics(self, key, db):
        """Use COMMON_ARCHIVER_METRICS to read from pg_stat_archiver as
        defined in 9.4 (first version to have this table).
        Uses a dictionary to save the result for each instance
        """
        # While there's only one set for now, prepare for future additions to
        # the table, mirroring _get_bgw_metrics()
        metrics = self.archiver_metrics.get(key)

        if self._is_9_4_or_above(key, db) and metrics is None:
            # Collect from only one instance. See _get_bgw_metrics() for details on why.
            sub_key = key[:2]
            if sub_key in self.db_archiver_metrics:
                self.archiver_metrics[key] = None
                self.log.debug(
                    "Not collecting archiver metrics for key: {0} as "
                    "they are already collected by another instance".format(key)
                )
                return None

            self.db_archiver_metrics.append(sub_key)

            self.archiver_metrics[key] = dict(self.COMMON_ARCHIVER_METRICS)
            metrics = self.archiver_metrics.get(key)

        if not metrics:
            return None

        return {'descriptors': [], 'metrics': metrics, 'query': "select %s FROM pg_stat_archiver", 'relation': False}