opts.go
func Levels(levels ...logrus.Level) Option {
	return func(sh *StackdriverHook) error {
		sh.levels = levels
		return nil
	}
}
func Levels(levels ...logrus.Level) Option {
	return func(sh *StackdriverHook) error {
		sh.levels = levels
		return nil
	}
}
// Levels is an option that sets the logrus levels that the StackdriverHook
// will create log entries for.