MySQL Replication: Using HAProxy and an HTTP-Check

I read the Github engineering article on MySQL Replication with HAProxy. One paragraph of note to me is:

In this situation, HAProxy no longer uses a mysql-check but rather an http-check. The MySQL backend server provides an HTTP interface, responding with HTTP 200 or HTTP 503 depending on replication lag. HAProxy will interpret these as “good” (UP) or “bad” (DOWN), respectively.

I am not aware of any MySQL HTTP Service. Is this a custom script that I have to write to provide this type of http-check?