code: func Async(check Check, interval time.Duration) Check {
  	return AsyncWithContext(context.Background(), check, interval)
  }
docs: // Async converts a Check into an asynchronous check that runs in a background
  // goroutine at a fixed interval. The check is called at a fixed rate, not with
  // a fixed delay between invocations. If your check takes longer than the
  // interval to execute, the next execution will happen immediately.
  //
  // Note: if you need to clean up the background goroutine, use AsyncWithContext().