docs: def _get_connection(self, key, host='', port='', user='', password='', database_url='', tags=None, use_cached=True):
          "Get and memoize connections to instances"
          if key in self.dbs and use_cached:
              return self.dbs[key]
          try:
              connect_kwargs = self._get_connect_kwargs(
                  host=host, port=port, user=user, password=password, database_url=database_url
              )
  
              connection = pg.connect(**connect_kwargs)
              connection.set_isolation_level(pg.extensions.ISOLATION_LEVEL_AUTOCOMMIT)
  
          # re-raise the CheckExceptions raised by _get_connect_kwargs()
          except CheckException:
              raise
  
          except Exception:
              redacted_url = self._get_redacted_dsn(host, port, user, database_url)
              message = u'Cannot establish connection to {}'.format(redacted_url)
  
              self.service_check(
                  self.SERVICE_CHECK_NAME,
                  AgentCheck.CRITICAL,
                  tags=self._get_service_checks_tags(host, port, database_url, tags),
                  message=message,
              )
              raise
  
          self.dbs[key] = connection
          return connection