[C++] Add rows scanned to open telemetry / profiling

It's not described at https://duckdb.org/dev/profiling but this is included in DuckDB's profiling and was helpful in looking at their scans to see the benefits of predicate push down

**Reporter**: [Jonathan Keane](https://issues.apache.org/jira/browse/ARROW-15529) / @jonkeane
#### Related issues:
- [[C++] Add OT spans for the scanner](https://github.com/apache/arrow/issues/30581) (is blocked by)

_{**Note**: *This issue was originally created as [ARROW-15529](https://issues.apache.org/jira/browse/ARROW-15529). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start with the related issue, “Add OT spans for the scanner,” since this issue is blocked by it, and review the DuckDB profiling page linked in the report. Identify where Arrow’s scanner telemetry or profiling data is exposed and how rows scanned should be represented; done means that metric is available for scan analysis and described in the profiling documentation.

Written by the indexing model from the issue text.