[8.14](backport #39173) [add_session_metadata processor] Keep exited processes in the process DB

[mergify]

Proposed commit message

With the add_session_metadata processor, don't remove processes from the process db when the process has exited.

The processor can be run on an fork/exec events after the process has actually exited, so the process must remain in the DB after it has exited, so the info can be used in enrichment of these events.

Now that the process is kept in the DB, the exit code is also appended on exit events, so the exit code can be used in enrichment of the exit events.

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.

Related issues

• Closes [Security-Linux Platform Integrations] Auditbeat missing Session View Process fields  #38955

---

This is an automatic backport of pull request #39173 done by [Mergify](https://mergify.com).

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[elasticmachine]

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• Start Time: 2024-04-25T20:29:03.210+0000

• Duration: 61 min 20 sec

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Total	533

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