[7.x](backport #28371) [Metricbeat] gcp.pubsub: uniform metricset's fields

[mergify]

This is an automatic backport of pull request #28371 done by Mergify.
Cherry-pick of 5cc5d0f has failed:

On branch mergify/bp/7.x/pr-28371
Your branch is up to date with 'origin/7.x'.

You are currently cherry-picking commit 5cc5d0f641.
  (fix conflicts and run "git cherry-pick --continue")
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Changes to be committed:
	modified:   x-pack/metricbeat/module/gcp/fields.go
	modified:   x-pack/metricbeat/module/gcp/pubsub/_meta/fields.yml
	modified:   x-pack/metricbeat/module/gcp/pubsub/pubsub_integration_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   metricbeat/docs/fields.asciidoc

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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