[receiver/couchdb] Enable re-aggregation feature

[dmitryax]

Part of #45396.

Enable the re-aggregation feature for the couchdb receiver by:

1. Setting reaggregation_enabled: true in metadata.yaml
2. Ensuring all attributes are properly defined with appropriate levels:
   • required: cannot be reaggregated
   • recommended: enabled by default but can be disabled by users
   • opt_in: disabled by default but can be enabled by users
3. Validate the feature after the feature gate is enabled: all existing tests pass, manual check

[github-actions]

Pinging code owners:

• receiver/couchdb: @antonblock

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[Amaan729]

Hi! I'd like to work on this. I've opened PR #47648 enabling the re-aggregation feature for the couchdb receiver — adds reaggregation_enabled: true to metadata.yaml, marks all attributes as requirement_level: recommended, and regenerates all derived files. Tests pass. Happy to iterate based on feedback.