[Metricbeat] gcp.billing: uniform metricset's fields

[endorama]

What does this PR do?

Uniform gcp.billing fields.yml file.

Why is it important?

As part of #27974 this PR uniform gcp.billing metricset fields.

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.

Author's Checklist

• Run tests TEST_TAGS=gcp MODULE=gcp mage goIntegTest cc @kaiyan-sheng
• updated fields.go before merging

How to test this PR locally

Related issues

• Required for [Metricbeat] gcp: respect Beats naming conventions #27974

Use cases

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Logs

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Pinging @elastic/integrations (Team:Integrations)

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Build stats

• Start Time: 2021-10-12T14:48:24.877+0000

• Duration: 67 min 15 sec

• Commit: c06998f

Test stats 🧪

Test	Results
Failed	0
Passed	2696
Skipped	248
Total	2944

💚 Flaky test report

Tests succeeded.

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