Add do.Once protection to beater interface

[fearful-symmetry]

What does this PR do?

This is a fix for #32608, which in turn I'm hoping might fix some of the issues with the pidfile fix here: #31670 (comment)

This wraps the beater Stop() call used by the beats in a once.Do() statement in order to prevent a "close of closed channel" panic. This appears to happening under agent, presumably due to a SIGINT handler calling Stop() at the same time another component is shutting down and also trying to call Stop(). There's no added changes for auditbeat, since it just imports the metricbeat beater interface.

I've tested this by prodding elastic-agent with a restart command, can't get the panic again.

Why is it important?

We don't want beats to randomly panic.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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Pinging @elastic/elastic-agent (Team:Elastic-Agent)

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @fearful-symmetry? 🙏.
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: 2022-12-07T20:40:31.666+0000

• Duration: 36 min 34 sec

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[cmacknz]

Do need to make this change for x-pack/osquerybeat? That's the only beat I don't see touched or explained already.

[fearful-symmetry]

@cmacknz osquerybeat has its own protections built in:

beats/x-pack/osquerybeat/beater/osquerybeat.go

Line 109 in 951b244

func (bt *osquerybeat) close() {

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[fearful-symmetry]

/test