chore: [k209] fix(sharding): use without() grouping when merging avg_over_time shard results

[grafanabot]

Backport eb8a363 from #12176

---

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
  • If the change is worth mentioning in the release notes, add add-to-release-notes label
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_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
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR