[8.x](backport #41022) [AWS] Use namespace for GetListMetrics when exists

[mergify]

Proposed commit message

This PR is to change GetListMetricsOutput function (which calls the ListMetrics API) to use namespaces when namespaces are given in AWS CloudWatch configuration in Metricbeat.

Instead of returning all possible metrics from CloudWatch from ALL namespaces and filter later, we only need to query the ListMetrics API to get the metrics under the namespaces that we care about.

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.

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This is an automatic backport of pull request #41022 done by [Mergify](https://mergify.com).

[botelastic]

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