Backport PR #17185 to 8.18: Fix empty node stats pipelines

[github-actions]

Backport PR #17185 to 8.18 branch, original message:

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Release notes

Fixed an issue where the /_node/stats API displayed empty pipeline metrics when X-Pack monitoring was enabled

What does this PR do?

When monitoring is enabled, the metrics store does not initialise data for monitoring pipeline. The api has null pointer when fetching the pipeline config like workers, batch_size.

Why is it important/What is the impact to the user?

/_node/stats gives empty pipelines: {}

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• closes: Node Stats API doesn't return pipelines when Internal Monitoring is Enabled #17180

Use cases

Screenshots

Logs

[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

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• Commit: 2cb2bc4

Failed CI Steps

• 🥼 Integration Tests / part 1