fix: allow any level for aggregated metrics

[trevorwhitney]

What this PR does / why we need it:

It was a mistake to assume that level values would match one of the defined constant options, and not worth the performance gain of pre-allocating the map to eliminate all the other options.

This change supports any level for aggregated metrics.

Which issue(s) this PR fixes:
Fixes #14213

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

[ashwanthgoli]

LGTM