fix: always write detected_level when enabled, even if unknown

[trevorwhitney]

What this PR does / why we need it:

Some aggregated metric lines are missing detected_level because when level is unknown we don't write detected_level to structured metadata. This PR offers a different approach to fix this than #14463, and it is to always write detected_level structured metadata when level detection is enabled, even if it's unknown.

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
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[shantanualsi]

LGTM. Thanks!