chore: [k218] fix: enable service detection for otlp endoint

[grafanabot]

Backport 4f962ef from #14036

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What this PR does / why we need it:

#13702 moved service detection further up in the distributor logic, from here to the push request parsing here. The problem with that is we pass multiple push request parsers to that pushHandler, so now service detection was limited to only in one of them, the traditional push path. This PR adds service detection to the OTLP parser, so it happens at the same time in the distributors logic (honoring the spirit of #13702), but now doing so for OTLP pushes as well.

Which issue(s) this PR fixes:
Fixes #14035

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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