retry.go
func (rs *RequestState) SinceStart(now time.Time, fallback time.Duration) time.Duration {
	if rs == nil {
		return fallback
	}
	return now.Sub(rs.Start)
}
func (rs *RequestState) SinceStart(now time.Time, fallback time.Duration) time.Duration {
	if rs == nil {
		return fallback
	}
	return now.Sub(rs.Start)
}
// SinceStart returns the time since the start of the request. If there is no request state,
// then the fallback is returned.