docs: def in_flight_request_count(self, node_id=None):
          """Get the number of in-flight requests for a node or all nodes.
  
          Arguments:
              node_id (int, optional): a specific node to check. If unspecified,
                  return the total for all nodes
  
          Returns:
              int: pending in-flight requests for the node, or all nodes if None
          """
          if node_id is not None:
              conn = self._conns.get(node_id)
              if conn is None:
                  return 0
              return len(conn.in_flight_requests)
          else:
              return sum([len(conn.in_flight_requests)
                          for conn in list(self._conns.values())])