[8.14](backport #39940) x-pack/filebeat/input/cel: fix handling of infinite values in rates

[mergify[bot]]

Proposed commit message

The protobuf.Value.AsInterface method encodes +Inf as "Infinity" so ensure that we accept that string.

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• [ ] 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.

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Related issues

• For elastic/integrations#10185

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This is an automatic backport of pull request #39940 done by Mergify.

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