When fetching label names, the API should return their cardinality

[grafakus]

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

When fetching label names from the /querier.v1.QuerierService/LabelNames API endpoint, it would be great if the API could return the labels cardinality.

This would enable (e.g.) a UI to adapt itself and to prevent long loading times, or even timeouts, when fetching label values. Another advantage would be for DataTrails-like UI, where a threshold could be used to determine which timeseries ("grouped by") to automatically display or not.

Describe the solution you'd like

See above.

Describe alternatives you've considered

The Grafana app plugin currently hides high cardinality labels, based on their names. The query builder has also some special logic to prevent fetching label values in this case.

Additional context

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