docs: def _get_instance_metrics(self, key, db, database_size_metrics, collect_default_db):
          """
          Add NEWER_92_METRICS to the default set of COMMON_METRICS when server
          version is 9.2 or later.
  
          Store the list of metrics in the check instance to avoid rebuilding it at
          every collection cycle.
  
          In case we have multiple instances pointing to the same postgres server
          monitoring different databases, we want to collect server metrics
          only once. See https://github.com/DataDog/dd-agent/issues/1211
          """
          metrics = self.instance_metrics.get(key)
  
          if metrics is None:
              host, port, dbname = key
              # check whether we already collected server-wide metrics for this
              # postgres instance
              if self._server_known(host, port):
                  # explicitly set instance metrics for this key to an empty list
                  # so we don't get here more than once
                  self.instance_metrics[key] = []
                  self.log.debug(
                      "Not collecting instance metrics for key: {} as "
                      "they are already collected by another instance".format(key)
                  )
                  return None
              self._set_server_known(host, port)
  
              # select the right set of metrics to collect depending on postgres version
              if self._is_9_2_or_above(key, db):
                  self.instance_metrics[key] = dict(self.COMMON_METRICS, **self.NEWER_92_METRICS)
              else:
                  self.instance_metrics[key] = dict(self.COMMON_METRICS)
  
              # add size metrics if needed
              if database_size_metrics:
                  self.instance_metrics[key].update(self.DATABASE_SIZE_METRICS)
  
              metrics = self.instance_metrics.get(key)
  
          # this will happen when the current key contains a postgres server that
          # we already know, let's avoid to collect duplicates
          if not metrics:
              return None
  
          res = {
              'descriptors': [('psd.datname', 'db')],
              'metrics': metrics,
              'query': "SELECT psd.datname, %s "
              "FROM pg_stat_database psd "
              "JOIN pg_database pd ON psd.datname = pd.datname "
              "WHERE psd.datname not ilike 'template%%' "
              "  AND psd.datname not ilike 'rdsadmin' "
              "  AND psd.datname not ilike 'azure_maintenance' ",
              'relation': False,
          }
  
          if not collect_default_db:
              res["query"] += "  AND psd.datname not ilike 'postgres'"
  
          return res