fix(aggregated_metrics): Fix the IsError method causing retries (backport k231)

[loki-gh-app]

Backport 7a7b3b1 from #15296

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What this PR does / why we need it:
The IsError method does not return true even though status is 204. This is causing retries that makes aggregated metrics incorrect.
This PR fixes it and adds some tests.

#incident-2024-12-05-metric-aggregation-is-broken

Which issue(s) this PR fixes:
Fixes #

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
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