Add copy_from option to the Append processor

[chrisberkhout]

Summary

A copy_from option for the Append processor, matching the option in the Set processor.

This makes it possible to refer to existing non-String values.

Discussion

The alternative is to use a Script processor, which requires Painless code to manually initialize the destination field and to avoid duplicates if necessary.

One variation on this use case is here and in the following 7 script processors.

I'd be happy to add an ignore_empty_value option as well, which would make it easier to collect values that may exist in multiple locations into one destination.

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[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[elasticsearchmachine]

Hi @chrisberkhout, I've created a changelog YAML for you.

[github-actions]

🔍 Preview links for changed docs

• docs/reference/enrich-processor/append-processor.md

[andrewkroh]

Happy to have this feature to avoid templating.

This change will also need an accompanying modification to the elasticsearch-specification to add the new parameter at https://github.com/elastic/elasticsearch-specification/blob/e585438d116b00ff34643179e6286e402c0bcaaf/specification/ingest/_types/Processors.ts#L329-L344

[chrisberkhout]

This change will also need an accompanying modification to the elasticsearch-specification to add the new parameter at https://github.com/elastic/elasticsearch-specification/blob/e585438d116b00ff34643179e6286e402c0bcaaf/specification/ingest/_types/Processors.ts#L329-L344

Thanks. I opened a draft PR for it: elastic/elasticsearch-specification#5056