[Metricbeat] GCP refactor getFilterForMetric ifelse to switch

[gpop63]

What does this PR do?

Refactors getFilterForMetric if else cases to switch cases as requested by @endorama #32964 (comment)

Why is it important?

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.

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @gpop63? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2022-09-12T10:51:28.072+0000

• Duration: 51 min 4 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1316
Skipped	87
Total	1403

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[endorama]

This changeset does not require a changelog, as no functionality changes, so you could remove this.

[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-metrics_req_ifelse_to_switch upstream/gcp-metrics_req_ifelse_to_switch
git merge upstream/main
git push upstream gcp-metrics_req_ifelse_to_switch