Refactor the vllm specific code to become model server agnostic

[ahg-g]

Currently the epp hardcodes the metrics to vllm. To support other model servers, we should make this logic configurable via flags to allow integrations with any model server adhering to the model server protocol as defined in https://github.com/kubernetes-sigs/gateway-api-inference-extension/blob/main/docs/proposals/003-endpoint-picker-protocol/README.md

[ahg-g]

/assign @BenjaminBraunDev

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[BenjaminBraunDev]

Commenting to allow assignment!

[ahg-g]

/assign @BenjaminBraunDev