Migrate Netflow code to the logp logging library (targeting main)

[jrmolin]

This was created from #42270, which missed the 8.18 cutoff and had to be re-targeted to main. I'm bad at git rebase, so I just created a patch file and applied it to main. I have incorporated the comments from that PR, but will address any provided now.

Proposed commit message

[Netflow] Migrate from log to the logp logging library

• The rest of beats is using github.com/elastic/elastic-agent-libs/logp, so make Netflow use it, too.
• Cleaned up the test code as well

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

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Implementation Detail: Logging should use logp everywhere #42247

Use cases

Screenshots

Logs

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[elasticmachine]

Pinging @elastic/sec-deployment-and-devices (Team:Security-Deployment and Devices)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @jrmolin? 🙏.
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-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[jrmolin]

@andrewkroh Thanks! Could you make sure the labels for backport are correct, please?

Is it okay that the lints are failing?

[andrewkroh]

Those lint issues were pre-existing, and you can merge without those passing, but there's one that looks important that we should address separately. (Ideally, we would make all the checks pass in a standalone PR.)

This change supports a new feature that will go into 8.19. So I changed the backport labels to only target 8.x (which will become 8.19 in the future).

[andrewkroh]

error is not nil (line 159) but it returns nil (nilerr)

This linter warning isn't related to your change, but it does look like a real problem whereby we are ignoring an error. Can you open an issue for this and we can address it separately.

[jrmolin]

#42790