[Metricbeat] gcp.gke: uniform metric's fields

[endorama]

What does this PR do?

Uniform gcp.gke fields.yml file: flatten and sort fields, add TestFetch integration test and fix resulting missing fields.

Why is it important?

As part of #27974 this PR uniform gcp.gke metricset fields.

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

Author's Checklist

• Run tests TEST_TAGS=gcp MODULE=gcp mage goIntegTest
• updated fields.go before merging

How to test this PR locally

Related issues

• Required for [Metricbeat] gcp: respect Beats naming conventions #27974

Use cases

Screenshots

Logs

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Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2021-10-13T15:06:29.996+0000

• Duration: 112 min 35 sec

• Commit: 534efb4

Test stats 🧪

Test	Results
Failed	0
Passed	9538
Skipped	2537
Total	12075

💚 Flaky test report

Tests succeeded.

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[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b gcp/uniform-fields-gke upstream/gcp/uniform-fields-gke
git merge upstream/master
git push upstream gcp/uniform-fields-gke