docs: def reply_bytes(self, request):
          """Take a `Request` and return an OP_MSG message as bytes."""
          flags = struct.pack("<I", self._flags)
          payload_type = struct.pack("<b", 0)
          payload_data = bson.BSON.encode(self.doc)
          data = b''.join([flags, payload_type, payload_data])
  
          reply_id = random.randint(0, 1000000)
          response_to = request.request_id
  
          header = struct.pack(
              "<iiii", 16 + len(data), reply_id, response_to, OP_MSG)
          return header + data