func TestCounter(counter metrics.Counter, value func() float64) error {
	want := FillCounter(counter)
	if have := value(); want != have {
		return fmt.Errorf("want %f, have %f", want, have)
	}
	return nil
}
TestCounter puts some deltas through the counter, and then calls the value
func to check that the counter has the correct final value.
package teststat
import (
	"errors"
	"fmt"
	"math"
	"math/rand"
	"strings"
	"github.com/go-kit/kit/metrics"
)