Add summary stats and metrics for stats cache

[salvacorts]

What this PR does / why we need it:
When a query finishes, we return (and log) the following stats:

Cache.Chunk.Requests             0
Cache.Chunk.EntriesRequested     0
Cache.Chunk.EntriesFound         0
Cache.Chunk.EntriesStored        0
Cache.Chunk.BytesSent            0 B
Cache.Chunk.BytesReceived        0 B
Cache.Chunk.DownloadTime         0s
Cache.Index.Requests             0
Cache.Index.EntriesRequested     0
Cache.Index.EntriesFound         0
Cache.Index.EntriesStored        0
Cache.Index.BytesSent            0 B
Cache.Index.BytesReceived        0 B
Cache.Index.DownloadTime         0s
Cache.Result.Requests            13
Cache.Result.EntriesRequested    13
Cache.Result.EntriesFound        13
Cache.Result.EntriesStored       0
Cache.Result.BytesSent   0 B
Cache.Result.BytesReceived       2.5 kB
Cache.Result.DownloadTime        4.600266ms

In addition to that, we log the following in metrics.go:

level=info ts=2023-05-29T09:17:10.93029945Z caller=metrics.go:152 component=frontend org_id=145265 traceID=52d59b78fe6b9221 sampled=true latency=fast query="{cluster=\"dev-us-central-0\", namespace=~\"loki.*\", container=~\"distributor|ingester
|promtail|index-gateway|compactor\"} |= \"thislinewillnotexist\"" query_hash=1194136170 query_type=filter range_type=range length=3h0m0s start_delta=165h37m24.930289434s end_delta=162h37m24.930289612s step=43s duration=2.473055ms status=200 lim
it=30 returned_lines=0 throughput=0B total_bytes=0B lines_per_second=0 total_lines=0 total_entries=0 store_chunks_download_time=0s queue_time=0s splits=13 shards=0 cache_chunk_req=0 cache_chunk_hit=0 cache_chunk_bytes_stored=0 cache_chunk_bytes
_fetched=0 cache_chunk_download_time=0s cache_index_req=0 cache_index_hit=0 cache_index_download_time=0s cache_result_req=13 cache_result_hit=13 cache_result_download_time=4.600266ms

With the goal of being able to better monitor how the stats cache is performing; this PR adds stats for the index stats cache, similarly to how it's done for the results cache.

Here's an example of the new stats being returned and printed:

...
Cache.StatsResult.Requests               180
Cache.StatsResult.EntriesRequested       129
Cache.StatsResult.EntriesFound   129
Cache.StatsResult.EntriesStored          51
Cache.StatsResult.BytesSent              0 B
Cache.StatsResult.BytesReceived          75 kB
...

And the new stats from metrics.go

... caller=metrics.go:155 ... cache_stats_results_req=129 cache_stats_results_hit=129 cache_stats_results_download_ti
me=156.864429ms ...

Special notes for your reviewer:

• Blocked by Stats cache can be configured independently #9535
• Note the newstats.GetOrCreateContext func. It's used inside the query.Exec method so we don't overwrite the stats added in the stats middleware.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR