Update filetarget.go

[lozbrown]

Allow promtail to skip or stop tailing watching files/folders that contain a file with promtail_ignore in the title.

promtail works poorly with transient log files (like airflow dag logs) because it tried to watch an ever growing list of files folders which will likely never change again.

This allows you to stop promtail watching those files/folders on the fly

What this PR does / why we need it:

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
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