[V1][Metrics] Fix http metrics middleware

[markmc]

PR #15657 hooked up prometheus_fastapi_instrumentator but only for V0.

$ curl http://0.0.0.0:8000/metrics 2>/dev/null  | grep -P '^http_(?!.*(_bucket|_created|_sum)).*'
http_requests_total{handler="/v1/completions",method="POST",status="2xx"} 201.0
http_request_size_bytes_count{handler="/v1/completions"} 201.0
http_response_size_bytes_count{handler="/v1/completions"} 201.0
http_request_duration_highr_seconds_count 201.0
http_request_duration_seconds_count{handler="/v1/completions",method="POST"} 201.0

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[markmc]

Rebased to pick up #15892 which should fix some of the v1/entrypoints/llm/test_struct_output_generate.py failures

[simon-mo]

@markmc this looks good, can you rebase?

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @markmc.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork