fix: short circuit level detection when already detected

[trevorwhitney]

What this PR does / why we need it:

#16924 changed how the pattern ingester sends the detected level back into the distributors for aggregated metrics. However, the distributors weren't actually checking if detected_level was already set in structured metadata, and therefore was still trying to do it's own level detection. This worked against my test data locally, as the original services had indexed level fields that showed up in the aggregated metric log line, but this very much didn't work in many other places :(

For example, I broke our ops environment real good. See all the unknown levels, and when I started running the custom image on distrubtors to test my change.

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

[loki-gh-app]

This PR must be merged before a backport PR will be created.

[loki-gh-app]

This PR must be merged before a backport PR will be created.