chore: refactor detected fields handler

[trevorwhitney]

What this PR does / why we need it:

This refactors NewDetectedFieldsHandler to return a base.Handler instead of a base.Middleware since it no longer needs access to next, and is therefore not really a middle-ware.

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
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
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