DML with RETURNING statements do not return update count / affected rows

[olavloite]

Description

When executing a DML statement containing a RETURNING clause (e.g., INSERT INTO table ... RETURNING id), RecordsAffected returns 0 instead of the actual number of rows inserted/updated.

Root Cause

1. In Cloud Spanner, DML statements with a RETURNING clause are executed as queries (returning a row stream).
2. The Google Cloud Spanner Go client library (google-cloud-go) does not populate the RowCount field on query row iterators (unlike standard DML iterators).
3. The Go SQL proxy driver (go-sql-spanner) relies on it.RowCount to populate the ResultSetStats.RowCountExact field. Because this field is not set by the Go client for query iterators, the Go proxy receives a row count of 0 and propagates rowCountExact: 0 back to the C# ADO.NET client.
4. As a result, the C# client always receives 0 for RecordsAffected for DML statements with a returning clause.

Steps to Reproduce

1. Execute a DML returning statement using ExecuteReader or ExecuteReaderAsync.
2. Drain the reader completely.
3. Read the RecordsAffected property. It will be 0 instead of the expected row count.