Regex support of add_resource_metadata

[gizas]

PR to document regex support of add_resource_metadata

• Closes: [Kubernetes Provider] Allow wildcards or regex when specifying "include_labels" and "include_annotations" elastic-agent#3346

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @gizas? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[gizas]

@kilfoyle I had closed this one: elastic/elastic-agent#4097
Can you please merge this if no objection? I have no rights

[kilfoyle]

LGTM! 🚀