How about exposing byteSize function

[k8scat]

I want to use byteSize to compute the size of a message and decide whether to send it，because I meet an error while sending a big message

func (m *ProducerMessage) byteSize(version int) int {
	var size int
	if version >= 2 {
		size = maximumRecordOverhead
		for _, h := range m.Headers {
			size += len(h.Key) + len(h.Value) + 2*binary.MaxVarintLen32
		}
	} else {
		size = producerMessageOverhead
	}
	if m.Key != nil {
		size += m.Key.Length()
	}
	if m.Value != nil {
		size += m.Value.Length()
	}
	return size
}