[9.3](backport #48684) fix(x-pack/filebeat/input/cel): add descriptions and units to OTel metrics

[mergify]

Proposed commit message

Add OpenTelemetry metric metadata (descriptions and units) to all CEL
input metrics to enable proper context when viewing data in metrics
databases.

Units follow UCUM [1] standard codes:
- "s" for duration metrics (seconds)
- "{run}", "{batch}", "{event}" for count metrics

Descriptions follow otelhttp style (capitalized, ending with period).

References:

[1]: https://ucum.org/ucum
[2]: https://opentelemetry.io/docs/specs/semconv/general/metrics/

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. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

Related issues

• Relates Add OTEL metrics to cel input #47014

References

• https://opentelemetry.io/docs/specs/semconv/general/metrics/ (guidelines for OTel metrics)
• https://ucum.org/ucum

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This is an automatic backport of pull request #48684 done by [Mergify](https://mergify.com).

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