Bump fluent-plugin-prometheus to 2.0.2

[szymonpk]

Proposed changes

Metrics like emit_count, num_errors, and others are missing when 2.0.1
is used [1], [2].

Also, bump .ruby-version to 2.7.4. It is the same as in the docker
image. The previous version is incompatible with some gems used in the
repo.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

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• I have read the CONTRIBUTING doc
• I have read the CLA
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules