这个帖子里面大佬的回复解决了我的问题。
如下：
Using the “port” parameter, it becomes possible to use a different port to send health-checks. On some servers, it may be desirable to dedicate a port to a specific component able to perform complex tests which are more suitable to health-checks than the application. It is common to run a simple script in inetd for instance. This parameter is ignored if the “check” parameter is not set. See also the “addr” parameter.
server tidb-1 192.168.0.1:4000 port 10080 check ??
我的配置修改：添加port 10080参数指定探测端口
server tidb-71 192.168.241.71:4000 send-proxy check port 10080 inter 2000 rise 2 fall 3
server tidb-72 192.168.241.72:4000 send-proxy check port 10080 inter 2000 rise 2 fall 3
server tidb-73 192.168.241.73:4000 send-proxy check port 10080 inter 2000 rise 2 fall 3
haproxy有指定监测端口的方式：
http://docs.haproxy.org/1.7/configuration.html#5.2-port