fix: enable service detection for otlp endoint

[trevorwhitney]

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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[svennergr]

The len(discoverServiceName) > 0 guard is another way of turning of service detection, I guess? Would it make sense to add a new var somewhere before this line like shouldDisdoverService := len(discoverServiceName) > 0 and use that also here?

[trevorwhitney]

I created the the variable and reused where I could, not sure if the line #s in your links were off, but it I don't think this can replace the hasServiceName logic, I think we need both

[ashwanthgoli]

LGTM. left a couple of nits

[grafanabot]

This PR must be merged before a backport PR will be created.