Tsdb/index sampling

[owen-d]

This PR lays the groundwork for index sampling in TSDB. To do so, it creates a new struct,

type Stats struct {
	Streams uint64
	Chunks  uint64
	Bytes   uint64
	Entries uint64
}

A set of (user, from, through, matchers) will resolve to the approximation of underlying streams, chunks, bytes, and entries.

In the short term, this will enable dynamic sharding in the query planner. In the long term, we can expose this as an endpoint and use it to power a host of new features such as cardinality analysis and informing users how much data a query will touch before running it.

There will be a followup PR which wires this into the index-gateway grpc layer.

ref #5428

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
-               loki	-0.7%

[slim-bean]

LGTM