Display per-user metrics metadata

[juliusv]

To give users some amount of initial feebback, it'd be great for Cortex to show some per-user stats in the user's Cortex dashboard:

• number of metrics
• number of time series
• samples/s

This will require adding backend support for getting those stats from the ingesters. Probably the distributor will have to query all ingesters for their current metadata for a given user and then aggregate those stats for the UI.

[rade]

design

[juliusv]

See also weaveworks/service#844 (comment) about implementing the minimal version of UI feedback without requiring any backend changes.