[7.x](backport #28372) [Metricbeat] gcp.storage: uniform metricset's fields

[mergify]

This is an automatic backport of pull request #28372 done by Mergify.
Cherry-pick of 4229ede has failed:

On branch mergify/bp/7.x/pr-28372
Your branch is up to date with 'origin/7.x'.

You are currently cherry-picking commit 4229edeac4.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   x-pack/metricbeat/module/gcp/storage/_meta/fields.yml
	modified:   x-pack/metricbeat/module/gcp/storage/storage_integration_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   metricbeat/docs/fields.asciidoc
	both modified:   x-pack/metricbeat/module/gcp/fields.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[elasticmachine]

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• Start Time: 2021-10-14T17:49:09.500+0000

• Duration: 94 min 46 sec

• Commit: d6f4bf7

Test stats 🧪

Test	Results
Failed	0
Passed	9656
Skipped	2447
Total	12103

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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 mergify/bp/7.x/pr-28372 upstream/mergify/bp/7.x/pr-28372
git merge upstream/7.x
git push upstream mergify/bp/7.x/pr-28372

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)