code: func (p *Pusher) Collector(c prometheus.Collector) *Pusher {
  	if p.error == nil {
  		p.error = p.registerer.Register(c)
  	}
  	return p
  }
docs: // Collector adds a Collector to the Pusher, from which metrics will be
  // collected to push them to the Pushgateway. The collected metrics must not
  // contain a job label of their own.
  //
  // For convenience, this method returns a pointer to the Pusher itself.