Remove unregister_vllm_metrics no-op patch

[vraiti]

PLEASE FILL IN THE PR DESCRIPTION HERE.

Purpose

Previously, vLLM unregister_vllm_metrics would cause VLLMPrometheusMetrics to destroy vllm_omni:-namespaced metrics upon construction. This has been fixed with https://github.com/vllm-project/vllm/pull/42331, so the monkey-patch and subsequent warning

Test Plan

vLLM Version:
v0.22.0

vLLM-Omni Commit:
54a9d24

Test Result

Current /metrics:
metrics-before.txt

/metrics after patch:
metrics-after.txt

Server boot logs:
vllm-omni-logs-after.txt

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[hsliuustc0106]

lgtm

[hsliuustc0106]

fix ci https://buildkite.com/vllm/vllm-omni/builds/11448/list

[vraiti]

fix ci https://buildkite.com/vllm/vllm-omni/builds/11448/list

The referenced CI failures seem preexisting? My code doesn't touch anything to do with Diffusers SP

[hsliuustc0106]

CI triage for Buildkite build #11472: this looks introduced by this PR.

Evidence:

• The only failing step in #11472 is Simple · Other Test.
• The failing command is pytest -sv -m 'core_model and cpu' --ignore=tests/diffusion --ignore=tests/model_executor ....
• The failures are all in tests/metrics/test_prometheus.py: 8 failed, 1601 passed.
• The failed assertions are all missing vllm:omni_* Prometheus metric families/samples, e.g. vllm:omni_num_requests_running, vllm:omni_requests_success_total, and vllm:omni_e2e_request_latency_s_count.
• This PR changes only metrics-related code: removes the scoped unregister_vllm_metrics patch and changes METRIC_PREFIX from vllm:omni_ to vllm_omni:.
• The same test_prometheus.py failures were already present in earlier PR build #11448, so this is not from the later merge-from-main commit. Build #11448 also had unrelated diffusion sp marker collection errors, but #11472’s remaining failure is the metrics prefix/registration issue.

Likely fix: either update the Prometheus tests and verify the new vllm_omni: pipeline metrics are actually emitted, or keep the old vllm:omni_ prefix / scoped unregister behavior until the CI vLLM dependency definitely includes the upstream unregister fix.