feat(pyroscope.receive_http): ensure consistent service_name label handling

[marcsanmi]

PR Description

This PR ensures that pyroscope.receive_http component sets the service_name label consistently from __name__ to match Pyroscope's behavior. When service_name already exists, it sets app_name from __name__ instead.

Which issue(s) this PR fixes

Notes to the Reviewer

I decided to duplicate the service name logic in Alloy rather than create a shared abstraction because:

• The logic is simple and stable (see pkg/ingester/pyroscope/ingest_adapter.go lines 125-149)
• Both implementations use different label types (Pyroscope's LabelSet vs Prometheus' labels.Labels) and forcing an abstraction would add unnecessary complexity

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

[simonswine]

I am not entirely sure if that is fully "consistent" with at least my expectations.

I mix both types of APIs (pushv1/ingest). Now I have a relabel rule modifying service_name to e.g. let's say append -grafana-labs.

Now pyroscope.write will still use the old value in __name__ to write the name for the /ingest request.

I think maybe it is better if we "rename" the __name__ to service_name and then just before sending we "rename" it back

[simonswine]

Thanks for your work on this. LGTM