Differentiate between different label scopes

[simonswine]

We currently treat external labels (or series label), which are applying to whole profile as received, in a similar way as sample labels (or pprof labels).

This is at best confusing and at worst it leads to data loss, when invalid sample labels prevent a profile from being ingested or existing external labels get overwritten by sample labels with the same name.

I do think going forward we need treat those as completely separate labels, a bit like Otel Traces has resource and span attributes.

This problem will become also more pressing as we adopt the new Otel Profiles model, which also features "profile" and "location" attributes.

This will require substantial changes how we validate, distribute and query label.