TSDB/query-planning-groundwork

[owen-d]

This PR does a few things that will prepare us for better query planning using the new /loki/api/v1/index/stats API:

• Adds a MatcherGroups()[][]*labels.Matcher method to the SampleExpr interface so we can resolve the underlying sets of stream matchers for metric queries. This will allow us to shard different legs of binary operations independently based on how much data each leg operates on.
• Adds a ShardResolver interface with default implementation. This is how we'll thread through index sampling into the ast mapper.

type ShardResolver interface {
	Shards(expr syntax.Expr) (int, error)
}

• Codec support for the IndexStats{Request,Response} so we can use them within the frontend middleware chains.

ref #5428

[slim-bean]

very cool

[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	-1.2%
+               iter	0%
+            storage	0%
+           chunkenc	0%
-              logql	-0.2%
+               loki	0%

[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	0.3%
+            querier	0%
- querier/queryrange	-1.2%
+               iter	0%
+            storage	0%
+           chunkenc	0%
-              logql	-0.2%
+               loki	0%