func (e *expvarCollector) Describe(ch chan<- *Desc) {
	for _, desc := range e.exports {
		ch <- desc
	}
}
Describe implements Collector.
package prometheus
import (
	"encoding/json"
	"expvar"
)
type expvarCollector struct {
	exports map[string]*Desc
}
func NewExpvarCollector(exports map[string]*Desc) Collector {
	return &expvarCollector{
		exports: exports,
	}
}