Expose metrics for number of active series queried within a given time period #7151
Description
Is your feature request related to a problem? Please describe.
As a Cortex metrics platform operator, I want to understand for tenants in Cortex, the percentage of active time-series ingested that were actually queried. Queried time series can be tracked using different time windows such as last 5m, 30m, 2h, etc.
Describe the solution you'd like
This metric doesn't need to be exact so some sort of probablistic data structure can be used to estimate cardinality like hyperloglog. Besides, sampling can be introduced to sample certain queries.
Describe alternatives you've considered
Should this metric be tracked in queries or ingesters?
It makes the most of sense to track recent data so I think cold series queried should be ignored. But for data in Ingester, should we only keep track of series queried from Head?
Additional context
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