ci: run prometheus regtest as final step for ASAN builds

[Copilot]

ASAN builds include USE_PROMEX=1 but had no dedicated final step to exercise the prometheus exporter under the sanitizer. This adds an explicit prometheus query at the end of ASAN CI runs to catch memory errors that only surface during prometheus scrapes.

Changes

• .github/workflows/vtest.yml: Add a new step "Run Prometheus query (ASAN only)" conditioned on contains(matrix.name, 'ASAN'), placed after the main reg-tests step:

- name: Run Prometheus query (ASAN only)
  if: ${{ contains(matrix.name, 'ASAN') }}
  run: |
    make reg-tests VTEST_PROGRAM=../vtest/vtest REG_TEST_FILES=reg-tests/contrib/prometheus.vtc

Uses REG_TEST_FILES to run only reg-tests/contrib/prometheus.vtc — no full suite re-run. The contains(matrix.name, 'ASAN') condition is already used elsewhere in the same workflow (line 73) for ASAN detection.

Original prompt

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This section details on the original issue you should resolve

<issue_title>invoke prometheus in regtests</issue_title>
<issue_description>if haproxy is built using address sanitizer we want to include additional step to regtests which performs a query to prometheus as a final step of test</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes invoke prometheus in regtests #23

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