opts.go
func Labels(labels map[string]string) Option {
	return func(sh *StackdriverHook) error {
		sh.labels = labels
		return nil
	}
}
func Labels(labels map[string]string) Option {
	return func(sh *StackdriverHook) error {
		sh.labels = labels
		return nil
	}
}
// Labels is an option that sets the labels to send with each log entry.