Preserve annotations in a kubernetes namespace metadata

[atercattus]

What does this PR do?

When kubernetes is used as autodiscover provider, we can use metadata in processors of filebeat.
But after #16834 we lost access to 'annotations' section. This PR fix it.

Why is it important?

We have custom tags in namespace annotations but have no access to them from the latest versions of filebeat. But we still have access to labels what is contradictory.

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.

How to test this PR locally

As usual: go test ./...

Related issues

• Relates Fix k8s metadata issue #16834

Use cases

kind: Namespace
metadata:
  annotations:
    sage/group: foo_bar # We lost this without fix

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💚 CLA has been signed

[elasticmachine]

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• Start Time: 2021-07-27T06:44:21.043+0000

• Duration: 133 min 47 sec

• Commit: d600c39

Test stats 🧪

Test	Results
Failed	0
Passed	49549
Skipped	5329
Total	54878

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Test stats 🧪

Test	Results
Failed	0
Passed	49549
Skipped	5329
Total	54878

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Pinging @elastic/integrations (Team:Integrations)

[ChrsMark]

lgtm, thanks for fixing this!

[ChrsMark]

/test