func (rtt *RoundTripTime) RoundTripDuration() time.Duration {
	return rtt.end.Sub(rtt.begin)
}
RoundTripDuration gives the total time spent in this roundtrip
package http
import "time"
func NewRoundTripTime() *RoundTripTime {
	rtt := &RoundTripTime{
		begin: time.Now(),
		end:   time.Now(),
	}
	return rtt
}
func (rtt *RoundTripTime) StartTracking() {
	rtt.begin = time.Now()
}
func (rtt *RoundTripTime) StopTracking() {
	rtt.end = time.Now()
}
func (rtt *RoundTripTime) StartTime() time.Time {
	return rtt.begin
}
func (rtt *RoundTripTime) StopTime() time.Time {
	return rtt.end
}