Provide API to access profiling data volume per label.

[simonswine]

Is your feature request related to a problem? Please describe.

Currently is really difficult to understand what series have which profiling volume.

Describe the solution you'd like

Would like to provide an API, that can based on TSDB index information, determine how much profiling data has been stored.

A potential returned unit could be for example the profiling sample count. (Providing bytes, there might be tricky because of deduplication of symbols), maybe ingested uncompressed bytes would be best as that is how we charge in Grafana Cloud.