receiver/kafka/kafka.go
func (o *Offset) MarshalText() ([]byte, error) {
	switch *o {
	case OffsetNewest:
		return []byte("newest"), nil
	case OffsetOldest:
		return []byte("oldest"), nil
	}
	return []byte(fmt.Sprintf("%v", *o)), nil
}
func (o *Offset) MarshalText() ([]byte, error) {
	switch *o {
	case OffsetNewest:
		return []byte("newest"), nil
	case OffsetOldest:
		return []byte("oldest"), nil
	}
	return []byte(fmt.Sprintf("%v", *o)), nil
}
// MarshalText marshals offset. It's used to handle two special cases "newest" and "oldest".