code: func (p *promProducer) Describe(ch chan<- *prometheus.Desc) {
  	prometheus.NewGauge(prometheus.GaugeOpts{Name: "Dummy", Help: "Dummy"}).Describe(ch)
  }
docs: // Describe passes all the available stat descriptions to Prometheus; we
  // send a single 'dummy' stat description, and then create our actual
  // descriptions on the fly in Collect() below.
  // This is necessary because Describe is expected to yield all possible
  // metrics to the channel, and must therefore send at least one value.
  // For a full description see:
  // https://godoc.org/github.com/prometheus/client_golang/prometheus#Collector