[8.12] Regex support of add_resource_metadata (backport #834)

[mergify]

This is an automatic backport of pull request #834 done by Mergify.
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On branch mergify/bp/8.12/pr-834
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Changes to be committed:
	modified:   docs/en/ingest-management/processors/processor-add_kubernetes_metadata.asciidoc

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	both modified:   docs/en/ingest-management/elastic-agent/configuration/providers/kubernetes-provider.asciidoc

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[gizas]

@kilfoyle can you please just accept incoming changes and merge this as well? Sorry no access

[kilfoyle]

Will do! it's unfortunate that the restrictions are so tight on this repo now. Just let me know anytime you need anything merged.