[8.x](backport #41505) [filebeat][gcs] - Added input metrics

[mergify[bot]]

Type of change

• Enhancement

Proposed commit message

Added input metrics along with some basic tests.

Checklist

• [x] My code follows the style guidelines of this project
• [x] 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
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

NOTE

Ignore linting errors

Disruptive User Impact

None

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Closes #36640
• Relates #41107

Use cases

Screenshots

Logs

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This is an automatic backport of pull request #41505 done by Mergify.

[botelastic[bot]]

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