code: func TCPChecker(addr string, timeout time.Duration) health.Checker {
  	return health.CheckFunc(func() error {
  		conn, err := net.DialTimeout("tcp", addr, timeout)
  		if err != nil {
  			return errors.New("connection to " + addr + " failed")
  		}
  		conn.Close()
  		return nil
  	})
  }
docs: // TCPChecker attempts to open a TCP connection.