opts.go
func Logf(f func(string, ...interface{})) Option {
	return func(s *Sentinel) error {
		s.logf = f
		return nil
	}
}
func Logf(f func(string, ...interface{})) Option {
	return func(s *Sentinel) error {
		s.logf = f
		return nil
	}
}
// Logf is a sentinel option to set a logger.