code: func (cm *ProtoConsumerMessage) GetPrevValue(msg proto.Message) (prevValueExist bool, err error) {
  	prevVal := cm.ConsumerMessage.GetPrevValue()
  	if prevVal == nil {
  		return false, nil
  	}
  	err = cm.serializer.Unmarshal(prevVal, msg)
  	if err != nil {
  		return true, err
  	}
  	return true, nil
  }
docs: // GetPrevValue returns the previous value associated with the latest message.