Migrate Netflow code to the logp logging library

[jrmolin]

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

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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)

[andrewkroh]

The godoc for Defaults() says "log output is discarded".

With this line, it is hard to guarantee that is true because initialization order can affect logp.L(). The global logger may or may not be initialized at the time at which this call is executed.

I didn't look at how this is used, but maybe it's better to separate the logger from the decoder config concept. And require that users of the decoders pass their own parent *logp.Logger at construction time.