code: func defaultTags(c context.Context, ctl task.Controller, cfg *messages.BuildbucketTask) map[string]string {
  	if c != nil {
  		return map[string]string{
  			"builder":                 cfg.Builder,
  			"scheduler_invocation_id": fmt.Sprintf("%d", ctl.InvocationID()),
  			"scheduler_job_id":        ctl.JobID(),
  			"user_agent":              info.AppID(c),
  		}
  	}
  	return map[string]string{
  		"builder":                 "",
  		"scheduler_invocation_id": "",
  		"scheduler_job_id":        "",
  		"user_agent":              "",
  	}
  }
docs: // defaultTags returns map with default set of tags.
  //
  // If context is nil, only keys are set.