[MISC] Add lora requests to metrics

[coolkp]

This PR adds lora requests to log metrics. The metrics will be logged only when the lora is enabled. Here is an example:

# HELP vllm:lora_requests_info Running stats on lora requests waiting and under process.
# TYPE vllm:lora_requests_info gauge
vllm:lora_requests_info{max_lora="1",running_adapters="",waiting_adapters=""} 1.0

We plan to leverage this information for routing decisions in https://github.com/kubernetes-sigs/llm-instance-gateway

[github-actions]

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[comaniac]

Otherwise LGTM

[comaniac]

This seems always fixed? In this case can we don't dump this value?

[coolkp]

across multiple deployments its hard to get this value, helps determine how many loras can be fitted on the server. You are right, its definitely static right now, initialised at runtime and thats it. I considered moving it to separate info metric like the cache config info. But I think in future there maybe value in enabling dynamic adjustment of max lora, like base_model which is static right now.

[comaniac]

LGTM