[AWS Fargate] Added support for DesiredStatus and KnownStatus for ECS Tasks

[girodav]

What does this PR do?

This PR adds DesiredStatus and KnownStatus among the collected metrics for ECS Fargate. The values are collected at the task level, not at the container level. More info at: https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task-metadata-endpoint-v4.html

This PR also includes some minor refactoring and fixes for golangci-lint related errors that were present prior to this change.

Why is it important?

See #32077

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.

How to test this PR locally

Inside x-pack/metricbeat/module/awsfargate/task_stats

• go test to execute unit tests
• go test -tags=integration to execute integration tests

To test the end-to-end data flow, the easiest approach would be running amazon-ecs-local-container-endpoints and metricbeat in two Docker containers on the same network (more info available at this blog post from AWS)

Related issues

• Closes [Metricbeat] [AWS Fargate] Include 'DesiredStatus' and 'KnownStatus' fields with module/agent integration #32077
• Relates [Metricbeat] [AWS Fargate] Include 'DesiredStatus' and 'KnownStatus' fields with module/agent integration integrations#3576

[elasticmachine]

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• Start Time: 2022-07-19T15:28:16.323+0000

• Duration: 96 min 8 sec

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Total	4854

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[girodav]

/test

[zmoog]

LGTM! I added a few comments for a couple of nonblocking aspects.

[zmoog]

Why are those two fields no longer present in the data.json file?

[girodav]

I noticed the same but could not find the exact cause. That files was generated by executing go test -data -tags=integration inside task_stats