Fix the incorrect ordering for topk and bottomk in shardable queries

[harry671003]

What this PR does:

• This ports the sharding fix implemented in Thanos: Fix sorted queries do not produce sorted results for shardable queries thanos-io/thanos#6125
• If the root expr is topk or bottomk, the response will be concatenated without a global sort.
• If the AST contains sort or sort_desc, the response will be sorted by values.
• Else, the response will be sorted by labels.

Examples:

#	Query	Merge mechanism	Prometheus Link	Remarks
1	1 + sort(sum by (code) (http_requests_total) )	Merge and then sort by values	link	Prometheus sorts by value.
2	sort(topk by (code) (10, http_requests_total))	Merge and then sort by values	link	Prometheus sorts by value.
3	topk by (code) (10, http_requests_total)	Merge responses but don't sort	link	Prometheus doesn't sort.
4	topk(5, http_requests_total) by (code) + sort(http_requests_total)	Merge and then sort by values	link	Prometheus doesn't sort.
5	sort(http_requests_total) + topk(5, http_requests_total) by (code)	Merge and then sort by values	link	Prometheus sorts by value.

There is discrepancy for case 4 where Prometheus doesn't sort the responses but Thanos will sort by value.

I believe the above approach will give reasonable compatibility with Prometheus without increasing the complexity in the merging logic in the frontend.

Checklist

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

[yeya24]

Can we still keep the Bytes? It should have better performance than String.
https://github.com/cortexproject/cortex/pull/5095/files

[yeya24]

In Thanos we haven't ported this change. thanos-io/thanos@e97f342#diff-110f6c24237fc98439097095e83e5610352ca50eceb3e8fbb8eef9528c65a3a8R284

[yeya24]

Thanks!