docs: 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}