Add stats log when querying store gateway

[yeya24]

What this PR does:

Add series stats log in querier to log request stats going to each store gateway. This was added to Thanos recently thanos-io/thanos#6352.

Example:

ts=2023-06-01T16:29:19.133919Z caller=blocks_store_queryable.go:725 level=info msg="store gateway series request stats" instance=localhost:9008 queryable_chunk_bytes_fetched=47864 queryable_data_bytes_fetched=49149 blocks_queried=4 series_merged_count=11 chunks_merged_count=212 postings_touched=4 postings_touched_size_sum=192 postings_to_fetch=0 postings_fetched=4 postings_fetch_count=4 postings_fetched_size_sum=192 series_touched=44 series_touched_size_sum=3011 series_fetched=44 series_fetch_count=4 series_fetched_size_sum=0 chunks_touched=212 chunks_touched_size_sum=41419 chunks_fetched=212 chunks_fetch_count=4 chunks_fetched_size_sum=111767

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[yeya24]

I am thinking if we need to log stats per store gateway request or we only log the aggregated stats for all store gateway for a query. The previous option is more verbose but better to maybe identify query skews for different store gateways.

[friedrichg]

Thanks!
I think is fine without aggregation, we lose important detail with aggregation

[yeya24]

I will go ahead and merge the pr.