[Metricbeat] Google Cloud: Do not try to setup region filter on Cloudfunctions metrics

[sayden]

What does this PR do?

This PR solves a bug that appears when setting a region parameter in gcp.yml only when using CloudFunctions.

Some Google Cloud services fail when you request metrics filtering by region. Beats has faced the same situation when requesting PubSub or LoadBalancing metrics from Stackdriver (Stackdriver is the service requested in metrics metricset).

Requests to those services must be done explicitly without filtering by region. Unfortunately, this isn't very well documented in GCP docs (or it's well hidden 😄 )

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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Pinging @elastic/integrations-services (Team:Services)

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Build stats

• Build Cause: Pull request #24218 updated

• Start Time: 2021-02-24T21:33:48.874+0000

• Duration: 37 min 56 sec

• Commit: ba0103b

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Test	Results
Failed	0
Passed	2455
Skipped	216
Total	2671

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Test	Results
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Passed	2455
Skipped	216
Total	2671

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/test