[8.x](backport #41410) [add_session_metadata] Always use correct code for backend in use.

[mergify]

Proposed commit message

With the add_session_metadata processor, the config backend option and actual backend in use doesn't always match; the 'auto' option doesn't match a real backend (kernel_tracing, procfs). This fixes some logic so that when the 'auto' option is used, the processor will always follow the code path intended for whatever the actual backend is use is.

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.

Disruptive User Impact

None

How to test this PR locally

In the auditbeat config, set each of the available backend options for add_session_metadata processor ("auto", "kernel_tracking", "procfs"), and ensure the processor is enriching data correctly for each.

e.g., use this config

- module: auditd
  # Load audit rules from separate files. Same format as audit.rules(7).
  processors:
    - add_session_metadata:
        backend: "auto"

---

This is an automatic backport of pull request #41410 done by [Mergify](https://mergify.com).

[botelastic]

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