code: func (serv *v2MetadataService) TagAndAdd(diffID layer.DiffID, hmacKey []byte, meta V2Metadata) error {
  	meta.HMAC = ComputeV2MetadataHMAC(hmacKey, &meta)
  	return serv.Add(diffID, meta)
  }
docs: // TagAndAdd amends the given "meta" for hmac hashed by the given "hmacKey" and associates it with a layer
  // DiffID. If too many metadata entries are present, the oldest one is dropped.