func NewCounter(name string) *Counter {
	return &Counter{
		f: expvar.NewFloat(name),
	}
}
NewCounter creates an expvar Float with the given name, and returns an object
that implements the Counter interface.
package expvar
import (
	"expvar"
	"sync"
	"github.com/go-kit/kit/metrics"
	"github.com/go-kit/kit/metrics/generic"
)
type Counter struct {
	f *expvar.Float
}