code: func hkdfEncodeLabel(labelIn string, hashValue []byte, outLen int) []byte {
  	label := "tls13 " + labelIn
  
  	labelLen := len(label)
  	hashLen := len(hashValue)
  	hkdfLabel := make([]byte, 2+1+labelLen+1+hashLen)
  	hkdfLabel[0] = byte(outLen >> 8)
  	hkdfLabel[1] = byte(outLen)
  	hkdfLabel[2] = byte(labelLen)
  	copy(hkdfLabel[3:3+labelLen], []byte(label))
  	hkdfLabel[3+labelLen] = byte(hashLen)
  	copy(hkdfLabel[3+labelLen+1:], hashValue)
  
  	return hkdfLabel
  }
docs: // struct HkdfLabel {
  //    uint16 length;
  //    opaque label<9..255>;
  //    opaque hash_value<0..255>;
  // };