Add ShareGPT end-to-end tests

[diamondburned]

This improves our end-to-end test coverage a bit more.

[diamondburned]

Details on the include_usage change:

• vllm codebase officially casts this using Python's bool(), so "true" would've worked equally as True: https://github.com/vllm-project/vllm/blob/f186cfe75e452aeb76f5233da7392d51ee34d3ef/vllm/entrypoints/utils.py#L242
• Official OpenAI API documentation writes include_usage=True
• Add E2e testing for Prometheus Querying and Report Contents #413 includes this same fix as well.

[jjk-g]

/lgtm
/approve

[k8s-ci-robot]

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