fix(sharding): use without() grouping when merging avg_over_time shard results

[ashwanthgoli]

What this PR does / why we need it:
avg_over_time expression that uses either keep or label_format stage without any aggregation fails on main with the following error: aggregation operator '"sum"' without grouping

Given these expressions contain stages that could reduce the labelset, sharding converts it to the following form sum (sum_over_time()) / sum(count_over_time()), but vector aggregation expr evaluator always expects a grouping. This pr updates the avg_over_time shard mapper to use without() grouping for generating vector agg expr when it is not explicitly set.

example query to reproduce this error:

avg_over_time({container="query-frontend"} |= `metrics.go` | logfmt | keep splits | unwrap split [$__auto])

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
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[jeschkies]

Wow. That one is one me. Incredible that it took so long. Could you add another test that's checking the output. Eg the equivalence test?

[grafanabot]

This PR must be merged before a backport PR will be created.

[grafanabot]

This PR must be merged before a backport PR will be created.

[TennyZhuang]

It seems that the fix was not included in latest release.

loki, version 3.1.0 (branch: k207, revision: 935aee77)

Is there any workaround, either by modifying the query, or downgrade the server to a previous version?