[kayx23]

Why this PR is needed

The release/3.18 stream Prometheus test scrapes immediately after a session block. Worker metric flushing and privileged-agent cache refresh are asynchronous, so the expected 200 and 502 session series can be absent even though product behavior is correct. Both assertions failed again during the automatic rerun on #14081.

This backports only the relevant readiness pattern from #14092. The 3.18 test polls the exposition for the existing expected series instead of assuming the next block sees it immediately. Product code and metric assertions are unchanged.

#14081 should rebase after this prerequisite merges.

Validation

• one commit changing only t/stream-plugin/prometheus-metrics.t
• git diff --check origin/release/3.18...HEAD
• preserves the exact expected status, listener, node, and count assertions
• full CI is running

[kayx23]

Closing this PR to keep the Admin API migration scoped to documentation and CI environment maintenance. This test-only backport was opened in response to an unrelated flaky check on a documentation PR; the documentation contract remains version-specific and is validated against its own branch sources.