[8.x] Fix empty node stats pipelines (backport #17185)

[mergify]

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

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This is an automatic backport of pull request #17185 done by [Mergify](https://mergify.com).

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
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[elasticmachine]

💚 Build Succeeded

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• Commit: e48f1b2

cc @kaisecheng