fix: expose PartitionedTopKExec metrics

[shinzoxD]

Which issue does this PR close?

• Closes PartitionedTopKExec does not expose any metrics (metrics() returns None) #24470

Rationale for this change

PartitionedTopKExec already collected ExecutionPlanMetricsSet internally, but ExecutionPlan::metrics() was unimplemented so it returned None. EXPLAIN ANALYZE therefore showed no metrics, and output_rows / output_batches could not be checked against what the operator actually emits.

What changes are included in this PR?

• Implement PartitionedTopKExec::metrics() so it returns the existing metrics set, matching SortExec / window operators.
• Record baseline output metrics (output_rows, output_batches, output_bytes) on the coalesced batches sent to the consumer, not on the pre-coalesce per-partition heap batches. elapsed_compute was already tracked via TopKMetrics.

This does not address #24468 (SortExec TopK output_batches).

Are these changes tested?

Added the issue's reproduction as partitioned_topk_output_batches_metric_matches_emitted_batches: it asserts metrics() is Some and that output_batches equals the number of batches actually emitted (5 coalesced batches of 10 rows).

Also ran:

• cargo test -p datafusion-physical-plan --lib topk (82 passed)
• cargo test -p datafusion --test core_integration partitioned_topk_output_batches_metric_matches_emitted_batches (passed)
• cargo fmt --all
• cargo clippy -p datafusion-physical-plan --all-targets --all-features -- -D warnings
• cargo clippy -p datafusion --test core_integration -- -D warnings

Are there any user-facing changes?

EXPLAIN ANALYZE now reports metrics for PartitionedTopKExec (output_rows, output_batches, elapsed_compute, etc.).

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[Jefffrey]

how does this differ from

• Fix topk emitted batches #24481

[Jefffrey]

for anyone looking at this PR please see my comment here:

• fix: roll back DiskManager usage when spill write I/O fails #24407 (comment)