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GH-39664: [C++][Acero] Ensure Acero benchmarks present a metric for identifying throughput (#40884)



### Rationale for this change

Acero benchmarks sometimes output metrics such as `items/s`, `bytes/s`, `batches/s`, and `rows/s`. However, there is inconsistency in how these metrics are presented across different benchmarks. We are undertaking an effort to standardize the output of these metrics to ensure uniformity and clarity in performance measurement across all Acero benchmarks. 

### What changes are included in this PR?

`rows/s` has a similar meaning to `items/s`.

- `bytes/s` and `items/s`: aggregate
- `bytes/s` and `rows/s`: asof_join
- `batches/s` and `rows/s`: project, filter, expression

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.

* GitHub Issue: #39664

Authored-by: Hyunseok Seo <hsseo0501@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>