code: func (p *Pusher) Gatherer(g prometheus.Gatherer) *Pusher {
  	p.gatherers = append(p.gatherers, g)
  	return p
  }
docs: // Gatherer adds a Gatherer to the Pusher, from which metrics will be gathered
  // to push them to the Pushgateway. The gathered metrics must not contain a job
  // label of their own.
  //
  // For convenience, this method returns a pointer to the Pusher itself.