App Engine Flexible Environment Java servlet getting too many /_ah/health requests

Question

I've followed the Java Quickstart for the App Engine Flexible Environment here:

https://cloud.google.com/appengine/docs/flexible/java/quickstart

Note the Servlet implementation there, which I've barely changed:

@SuppressWarnings("serial")
@WebServlet(name = "helloworld", value = "/" )
public class HelloServlet extends HttpServlet {

  @Override
  public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
    PrintWriter out = resp.getWriter();
    out.println("Hello, world - Flex Servlet");
  }
}

So all GET requests, including those from Google's Health Check thing for /_ah/health, will get a 200 response code. Despite this, when checking the logs, I see three requests every second for /_ah/health. These continue forever. Surely this is bad.

Here's my src/main/appengine/app.yaml:

runtime: java
env: flex
service: service-name
runtime_config:
   jdk: openjdk8
   server: jetty9
manual_scaling:
  instances: 1
handlers:
- url: /.*
  script: this field is required, but ignored
  secure: always
beta_settings:
  java_quickstart: true

How do I satisfy Health Check that my instance is healthy?