fix(metric_aggregation): Fix duplicate metrics registration (backport k230)

[loki-gh-app]

Backport 215f994 from #15142

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What this PR does / why we need it:

Fixes duplicate metric registration in metric aggregation in pattern ingesters. I used promauto.With(reg) along with m.reg.MustRegister(... that resulted in duplicate registration

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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