[8.14](backport #38953) Fix awscloudwatch worker allocation

[mergify]

Fix a bug in cloudwatch worker allocation that could cause data loss (#38918).

The previous behavior wasn't really tested, since worker tasks were computed in cloudwatchPoller's polling loop which required live AWS connections. So in addition to the basic logical fix, I did some refactoring to cloudwatchPoller that makes the task iteration visible to unit tests.

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
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Fixes awscloudwatch input drops data #38918

---

This is an automatic backport of pull request #38953 done by [Mergify](https://mergify.com).

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[elasticmachine]

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