code: func (e expvarCollector) Collect(ch chan<- prometheus.Metric) {
  	expvar.Do(func(kv expvar.KeyValue) {
  		collector{
  			f: func(m prometheus.Metric) {
  				ch <- m
  			},
  			labelValues: []string{kv.Key},
  			descs:       e.descs,
  		}.collectVar(kv.Value)
  	})
  }
docs: // Collect implements Collector.