[7.x](backport #26824) [Metricbeat] gcp: add GKE metricset

[mergify]

This is an automatic backport of pull request #26824 done by Mergify.

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[elasticmachine]

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• Start Time: 2021-08-20T08:41:09.188+0000

• Duration: 94 min 24 sec

• Commit: ee64e54

Test stats 🧪

Test	Results
Failed	0
Passed	9615
Skipped	2431
Total	12046

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Test stats 🧪

Test	Results
Failed	0
Passed	9615
Skipped	2431
Total	12046

[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 mergify/bp/7.x/pr-26824 upstream/mergify/bp/7.x/pr-26824
git merge upstream/7.x
git push upstream mergify/bp/7.x/pr-26824