[Prometheus] Fix pre-registration of handled metrics with context labels

[t-bowcock]

Changes

Build label list handledLabelsWithCode in loop to place code.String() in correct place.
Fixes #804

Verification

Tested with grpc server on a personal project.

[t-bowcock]

All unit tests passed when running make test locally. Not sure why it has failed here or how to re run the job

[johanbrandhorst]

I wouldn't worry about that test failure, there's no way this is related. Thanks for the fix!