[aws] Fix missing metrics bug when include_linked_accounts set to false

[kaiyan-sheng]

Proposed commit message

This PR is to fix a bug in cloudwatch metricbeat module when include_linked_accounts is false. Because where the wrongly put return, we are missing metrics in pagination. I also added a test for the case when there is more than one page of results from ListMetrics API.

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.

Disruptive User Impact

No disruptive user impact.

How to test this PR locally

1. Setup your AWS account to have multiple RDS clusters and make sure we have metrics in CloudWatch.
2. Start Metricbeat with aws.yml looks like this:

- module: aws
  period: 1m
  credential_profile_name: elastic-observability
  metricsets:
    - cloudwatch
  regions: eu-west-3
  latency: 10m
  include_linked_accounts: false
  metrics:
    - namespace: AWS/RDS
      name: ["CPUUtilization"]
      resource_type: rds
      statistic: ["Average"]

You should see metrics only being collected from 2 RDS clusters. But if you change the include_linked_accounts to true, then you will get metrics from all clusters (5 in this case).
3. Apply the changes from this PR and rebuild metricbeat locally. Run with the same aws.yml with include_linked_accounts equals to false and you should see metrics from all clusters now.

Related issues

• Closes [aws] missing metrics when include_linked_accounts set to false #40071

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @kaiyan-sheng? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

Pinging @elastic/obs-ds-hosted-services (Team:obs-ds-hosted-services)

[zmoog]

LGTM