code: func ComputeV2MetadataHMAC(key []byte, meta *V2Metadata) string {
  	if len(key) == 0 || meta == nil {
  		return ""
  	}
  	mac := hmac.New(sha256.New, key)
  	mac.Write([]byte(meta.Digest))
  	mac.Write([]byte(meta.SourceRepository))
  	return hex.EncodeToString(mac.Sum(nil))
  }
docs: // ComputeV2MetadataHMAC returns a hmac for the given "meta" hash by the given key.