Add an API to fetch information about native histogram active buckets

[krajorama]

There needs to be an API that can return what native histogram metrics (not series) are active and what are their series count and sum, average, min, max active bucket counts across their series.

The information would then be shown on a cardinality style dashboard. (out of scope for this issue)

The API should allow expansion to allow returning the information on a series level.

Proposed API endpoint: <prometheus-http-prefix>/api/v1/cardinality/active_native_histogram_buckets

Parameters:

• limit: optional, default to 20. Number of results to return.
• selector: optional, defaults to {__name__==".+"}
• level: optional, defaults to metric, can have the values metric or series.
• sortBy: optional, defaults to sum, can have the values sum, count, average, min, max and refers to the information returned.

Should honor the header Sharding-Control: n, like the /api/v1/cardinality/active_series endpoint.

First step can exclude series level, no sorting, no limit, no sharding. Just return the information on metrics level.