[CI] update serverless test index

[kaisecheng]

update metricbeat .monitoring-logstash-8-mb to .monitoring-logstash-9-mb

wait until [1], [2] get merged

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: b6487a2

Failed CI Steps

• 🥼 IT Persistent Queues / part 1

[mashhurs]

Change LGTM!

Some questions:

• One thing if we can validate: when upstream KB & ES changes get merged, serverless integration tests expectedly should fail, right? It would be great if we can validate first.
• Does serverless tests run on main only? I mean how are we covering 8.x?

[kaisecheng]

serverless integration tests expectedly should fail, right?

Yes. good suggestion

Does serverless tests run on main only? I mean how are we covering 8.x?

Yes, main only. You are right. We should cover 8x. Let me create an issue to track it.

[kaisecheng]

follow-up for testing 8.x #17154

[consulthys]

@kaisecheng the PRs above have been merged

[mergify]

This pull request does not have a backport label. Could you fix it @kaisecheng? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label.