code: func (j *AuroraJob) AddLabel(key string, value string) Job {
  	if _, ok := j.metadata[key]; ok {
  		j.metadata[key].Value = value
  	} else {
  		j.metadata[key] = &aurora.Metadata{Key: key, Value: value}
  		j.jobConfig.TaskConfig.Metadata = append(j.jobConfig.TaskConfig.Metadata, j.metadata[key])
  	}
  	return j
  }
docs: // Adds a Mesos label to the job. Note that Aurora will add the
  // prefix "org.apache.aurora.metadata." to the beginning of each key.