Cherry-pick #26412 to 7.x: [Metricbeat] Change Account ID to Project ID in gcp.billing module

[kaiyan-sheng]

Cherry-pick of PR #26412 to 7.x branch. Original message:

What does this PR do?

Currently the GCP Project ID is being ingested as cloud.account.id instead of cloud.project.id. This PR fixes that.

Why is it important?

See above

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

• [ ]

How to test this PR locally

Related issues

• Closes GCP billing reports wrong account ID  #26357

Use cases

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• Build Cause: Started by user kaiyan-sheng

• Start Time: 2021-06-28T13:28:47.812+0000

• Duration: 159 min 10 sec

• Commit: ed1a93b

Test stats 🧪

Test	Results
Failed	0
Passed	1969
Skipped	192
Total	2161

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Test stats 🧪

Test	Results
Failed	0
Passed	1969
Skipped	192
Total	2161