[serve] Add healthz endpoint for HttpProxy.

[fishbone]

Why are these changes needed?

This PR add an endpoint for http proxy for health check. This feature is critical for serve operator because serve service needs to add the node to the service endpoint in a dynamic way and it'll use this endpoint for health check and add the health one.

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[edoakes]

@iycheng I think by standard convention this should just be /healthz?

[fishbone]

@iycheng I think by standard convention this should just be /healthz?

@edoakes I put it in -/ because it's kind of in a different namespace. For example, user can still add /healthz for their usecase.

But it's your call. I'm ok to move it to /healthz.

[simon-mo]

/-/healthz is perfect for now. Thanks @iycheng