code: func HostnameGroupingKey() map[string]string {
  	hostname, err := os.Hostname()
  	if err != nil {
  		return map[string]string{"instance": "unknown"}
  	}
  	return map[string]string{"instance": hostname}
  }
docs: // HostnameGroupingKey returns a label map with the only entry
  // {instance="<hostname>"}. This can be conveniently used as the grouping
  // parameter if metrics should be pushed with the hostname as label. The
  // returned map is created upon each call so that the caller is free to add more
  // labels to the map.
  //
  // Deprecated: Usually, metrics pushed to the Pushgateway should not be
  // host-centric. (You would use https://github.com/prometheus/node_exporter in
  // that case.) If you have the need to add the hostname to the grouping key, you
  // are probably doing something wrong. See
  // https://prometheus.io/docs/practices/pushing/ for details.