[Metricbeat] support credentials_json in gcp module

[endorama]

What does this PR do?

Adds support for credentials_json metricset configuration parameter to gcp.billing and gcp.metrics metricset.

Why is it important?

This change is required to support at metricset level this parameter that is already supported in Kibana Integration UI.

Filebeat gcp inputs support this parameter and due to the limitations in Kibana is not possible to use input level variables but globals are required. This PR fixes this in order to prevent having a configuration parameter in the UI that is not used by inputs.

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

1. Build x-pack/metricbeat
2. Configure gcp module with:

  - module: gcp
    metricsets:
      - billing
    zone: "eu-"
    project_id: "..."
    credentials_json: '' # add your GCP SA JSON key content here
    exclude_labels: false
    dataset_id: "integration_testing_detached"
    table_pattern: "billing_export_test_data_detached"
    period: 24h

3. Add your credentials in credentials_json
4. Setup a BigQuery table with dummy data
5. Run metricbeat and see collected data, it should work

Related issues

• GCP package billing datastream tests integrations#2312

Use cases

Screenshots

Logs

[mergify]

This pull request does not have a backport label. Could you fix it @endorama? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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Build stats

• Start Time: 2022-01-25T16:43:43.561+0000

• Duration: 76 min 12 sec

• Commit: 6158887

Test stats 🧪

Test	Results
Failed	0
Passed	2731
Skipped	240
Total	2971

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