[AutoOps] Serialize custom struct before sending as event

[pickypg]

This pre-serialization step is unnecessary for AutoOps running as part of Metricbeat, but it is necessary to allow it to work with OTel because the OTel Consumer cannot handle the serialization of structs.

Proposed commit message

Pre-serialize AutoOps cat_shards metricset for OTel mode.

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
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

Wasted performance in Metricbeat mode, but required to work in OTel mode.

Author's Checklist

• Compile and run this code with [OTel] Support Serializing nested data #45008 (required for the inner array that needs to be converted to []any to make OTel happy)

How to test this PR locally

Merge this branch with #45008, then compile it with Elastic Agent and run it in OTel mode with this metricset.

Related issues

• Relates [OTel] Data Serialization fails on struct types #45005

[github-actions]

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[romulo-batista-elastic]

LGTM