fix: codec not initialized in downstream roundtripper

[kavirajk]

What this PR does / why we need it:
Looks like part of this PR introduced this bug. Root cause is codec is nil (never initialized) and panics during downstream Do.

Above patch, converted QueryFrontend tripperware into middleware but failed to pass the right codec to downstream roundtripper.

This PR passes the codec during InitFrontend.

Which issue(s) this PR fixes:
Fixes #12842

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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[kavirajk]

@rakheshkumbi can you verify this fixes your problem mentioned in #12842?

[slim-bean]

@kavirajk could you also make sure this gets backported into the 3.0.x branch?

[grafanabot]

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

[kavirajk]

@kavirajk could you also make sure this gets backported into the 3.0.x branch?

Sure @slim-bean

[rakheshkumbi]

@kavirajk Tested the fix, it works fine for me.