[Feature]: Expose a read-only API to check whether engine is sleeping

[waltforme]

🚀 The feature, motivation and pitch

The sleep feature works great!

We can do even better by adding a read-only probe to check whether the engine is sleeping or not. Today the /sleep and /wake_up API endpoints POSTs to the API server and mutates the state of the engine. We may introduce an /is_sleeping endpoint which GETs the sleep status.

There are at least two use cases asking for this feature.

1. We are doing orchestration and optimization among multiple instances of vLLMs, asleep or awake. We need a global state of all the instances' sleeping status. The proposed probe is necessary to compose this state, without having to be stateful, i.e. remember whether each and every engine has been put asleep.
2. Today a sleeping engine crashes if a request is sent to it. This probe will give a good citizen peace of mind before sending a request.

Alternatives

No response

Additional context

No response

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[youkaichao]

you should be able to manage the is_sleeping state from an upper level. after you post a /sleep call, you can mark that engine as sleeping.

I don't think we need to explicitly expose this API endpoint.