docs: add instruction to remove unwanted attributes from systemd logs

[sumo-drosiek]

This is feature our customer would like to use. WDYT about adding it to regular configuration as the following?

sumologic:
  logs:
    systemd:
      removeAttributes:
      - PRIORITY

Checklist

• Changelog updated or skip changelog label added
• Documentation updated
• Template tests added for new features
• Integration tests added or modified for major features

[andrzej-stencel]

I agree this should be possible without resorting to config.merge. I believe the configuration should allow allowlisting and denylisting attributes.

[sumo-drosiek]

I agree this should be possible without resorting to config.merge. I believe the configuration should allow allowlisting and denylisting attributes.

Allowlisting can be little bit complicated AFAIK we have to do it the following way:

1. copy/move allowlisted attributes to some temporary map
2. remove body
3. move temporary map to body

[swiatekm]

Can't we just add extraProcessors to the systemd pipeline, same as we have for containers?

[github-actions]

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