code: func TCPDialCheck(addr string, timeout time.Duration) Check {
  	return func() error {
  		conn, err := net.DialTimeout("tcp", addr, timeout)
  		if err != nil {
  			return err
  		}
  		return conn.Close()
  	}
  }
docs: // TCPDialCheck returns a Check that checks TCP connectivity to the provided
  // endpoint.