WIP Skip unmarshaling of exemplars when exemplars are disabled

[replay]

Skipping the unmarshaling of exemplars if a user has exemplars disabled anyway.

This would help us because we have users who have exemplars disabled and who are still sending requests with very large amounts of exemplars, right now we allocate buffers to decode all these exemplars just to then drop them.

The line number looks larger than the PR is, most of the lines are copy-pasted from generated files. I did not like having to copy-paste generated code, but I figured it's acceptable because we're already doing the same thing with the .Unmarshal() method of the type LabelAdapter here.