vllm only has /health endpoint

[kerthcet]

What would you like to be added:

We'll have startup, liveness and readiness probes for inference workloads, but vllm only supports /health endpoint, see discussion here: vllm-project/vllm#7078,

Why is this needed:

Completion requirements:

This enhancement requires the following artifacts:

• Design doc
• API change
• Docs update

The artifacts should be linked in subsequent comments.