func (c *Clock) StopAt(now time.Time) {
	c.observe("total", now.Sub(c.first))
}
StopAt reports the time difference between now and the time the clock was created at.

The metric produced by this method call will have a "stamp" tag set to
"total".
package stats
import "time"
type Clock struct {
	name  string
	first time.Time
	last  time.Time
	tags  []Tag
	eng   *Engine
}
func (c *Clock) Stamp(name string) {
	c.StampAt(name, time.Now())
}
func (c *Clock) StampAt(name string, now time.Time) {
	c.observe(name, now.Sub(c.last))
	c.last = now
}
func (c *Clock) Stop() {
	c.StopAt(time.Now())
}