code: func (msg *sentMessage) addPayloadSizeField(payload []byte) {
  	fieldSize := getSizeOfPayloadSizeField(payload)
  	field := make([]byte, 4)
  	binary.LittleEndian.PutUint32(field, uint32(len(payload)))
  	field = field[:fieldSize]
  	msg.Raw = append(msg.Raw, field...)
  	msg.Raw[0] |= byte(fieldSize)
  }
docs: // addPayloadSizeField appends the auxiliary field containing the size of payload