code: func GetGRPCHeader(md *metadata.MD, key string) string {
  	v := (*md)[key]
  	if len(v) < 1 {
  		return ""
  	}
  	return v[len(v)-1]
  }
docs: // GetGRPCHeader retrieves the last value found for a particular key. If key is
  // not found it returns an empty string.