Add aggregate-specific metrics to legacy grouped hash and TopK

[kosiew]

Which issue does this PR close?

• Part of Add aggregate specific metrics #23570

Rationale for this change

Aggregate-specific timing metrics should remain consistent across grouped aggregation implementations. Previously, queries using the legacy grouped hash or grouped TopK paths could expose less per-aggregate timing information than queries using the migrated grouped hash implementation, making metrics dependent on which internal aggregation path was selected.

This change extends aggregate-specific metrics to those remaining paths for the phases they actually perform.

What changes are included in this PR?

• Adds per-aggregate accumulator timing metrics to GroupedHashAggregateStream for update, merge, state, evaluate, and convert-to-state phases.
• Reuses the same aggregate labels and accumulator metric helpers used by the migrated grouped aggregation implementation.
• Adds per-aggregate argument evaluation timing to GroupedTopKAggregateStream.
• Documents that grouped TopK does not expose accumulator update, merge, state, or evaluate timing because it maintains MIN/MAX values directly in its priority map.
• Extends final-mode metric coverage to exercise both values of datafusion.execution.enable_migration_aggregate.

Are these changes tested?

Yes.

The patch adds test_legacy_groupby_aggregate_accumulator_metrics, which disables datafusion.execution.enable_migration_aggregate and verifies aggregate-specific arguments_time, update_time, and state_time metrics for the legacy grouped hash path, including positive update and state timings.

test_groupby_metrics_final_mode now exercises final-mode metrics with datafusion.execution.enable_migration_aggregate set to both true and false.

The patch also adds test_topk_aggregate_argument_metrics, which exercises grouped TopK aggregation and verifies that the per-aggregate agg_expr_0_arguments_time metric is present, labeled with MIN(a), and non-zero.

Are there any user-facing changes?

Yes. Query execution metrics now expose more consistent aggregate-specific timing information when grouped aggregation uses the legacy grouped hash or grouped TopK implementations.

There are no public API changes.

LLM-generated code disclosure

This PR includes LLM-generated code and comments. All LLM-generated content has been manually reviewed.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.97814% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.31%. Comparing base (cd8cef9) to head (120b2c8).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...hysical-plan/src/aggregates/grouped_topk_stream.rs	87.67%	2 Missing and 7 partials ⚠️
...ysical-plan/src/aggregates/group_values/metrics.rs	85.45%	2 Missing and 6 partials ⚠️
...hysical-plan/src/aggregates/grouped_hash_stream.rs	90.90%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #24523    +/-   ##
========================================
  Coverage   81.30%   81.31%            
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  Files        1117     1117            
  Lines      395798   396065   +267     
  Branches   395798   396065   +267     
========================================
+ Hits       321815   322049   +234     
- Misses      55164    55187    +23     
- Partials    18819    18829    +10     

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