Cherry-pick #18757 to 7.x: [Metricbeat] Fix getting compute instance metadata with partial zone/region config

[kaiyan-sheng]

Cherry-pick of PR #18757 to 7.x branch. Original message:

What does this PR do?

This PR fixes getting compute instance metadata when zone/region in googlecloud.yml is partial. For example:

- module: googlecloud
  metricsets:
    - compute
  region: "us-"
  project_id: "your project id"
  credentials_file_path: "your JSON credentials file path"
  exclude_labels: false
  period: 1m

This bug is introduced from #17913

Why is it important?

Instances.Get API does not accept partial zone name and Regions.Get API does not accept partial region name. So instead of using the region/zone information from the config, use the zone label from time series metric.

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.

[elasticmachine]

Pinging @elastic/integrations-platforms (Team:Platforms)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #18839 updated]

• Start Time: 2020-05-29T13:42:37.803+0000

• Duration: 66 min 50 sec

Test stats 🧪

Test	Results
Failed	0
Passed	497
Skipped	48
Total	545