Make sure k8s watchers close when closing k8s meta processor

[ChrsMark]

What does this PR do?

This PR ensures that k8s watchers are closed when the add_k8s_metadata processor is closed.

Why is it important?

To gracefully close the processor.
Most probably this is the reason of elastic/elastic-agent-autodiscover#41.

Fixes #35495

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @ChrsMark? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2023-05-31T15:30:32.257+0000

• Duration: 69 min 12 sec

Test stats 🧪

Test	Results
Failed	0
Passed	26417
Skipped	1975
Total	28392

💚 Flaky test report

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