fix: prevent panic when ContentLength is negative in Distributor RequestBodyTooLarge metrics (backport k249)

[loki-gh-app]

Backport 9e9f534 from #17054

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What this PR does / why we need it:

re:

• 1d99f4d

We encountered an edge case where Distributors panic when trying to increment their DiscardedBytes metrics for large request bodies:

panic: counter cannot decrease in value

Which issue(s) this PR fixes:

N/A

Special notes for your reviewer:

N/A

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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