Differentiate between different label scopes

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](https://github.com/open-telemetry/opentelemetry-proto/blob/main/opentelemetry/proto/profiles/v1development/profiles.proto) model, which also features "profile" and "location" attributes.

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