file: allow configuration of log filename extension

[efd6]

What does this PR do?

This adds the ability to configure the filename extension for logs in file.NewFileRotator and fixes a bug in the logging.

Why is it important?

Not all logs are ndjson and currently the debug log in NewFileRotator will not include the filename.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.md

Author's Checklist

• [ ]

Related issues

• For x-pack/filebeat/input/httpjson: add transaction tracer beats#32412

[elasticmachine]

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• Start Time: 2022-07-23T03:18:22.156+0000

• Duration: 9 min 20 sec

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Total	1084

🤖 GitHub comments

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[cmacknz]

I would log the extension with the filename in the debug log:

elastic-agent-libs/file/rotator.go

Lines 181 to 186 in 7bba8bb

	r.log.Debugw("Initialized file rotator",
	"filename", r.filename,
	"max_size_bytes", r.maxSizeBytes,
	"max_backups", r.maxBackups,
	"permissions", r.permissions,
	)

Otherwise LGTM.

[efd6]

@cmacknz I don't have write access on this repo. Would you merge please.