feat(structured-metadata-api): add structured metadata to /detected_fields API

[svennergr]

What this PR does / why we need it:

Followup of #13119. This PR adds structured metadata to the response of /detected_fields call. Structured metadata will be identified by an empty parsers array.

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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• 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

[cyriltovena]

LGTM

parseDetectedFields needs a big refactoring it should only need the iterator to detect fields and not stream.

cc @trevorwhitney so many allocations ! This does diminish the benefits of using a sketch.