spanner: expose commit stats

[danielnorberg]

Thank you a lot for this library, we're using it to interact with cloud spanner from our rust backend.

In the course of adding instrumentation we realized that it would be useful to have the mutation counts from our spanner transactions. It seems possible to set CommitOptions::return_commit_stats = true and then the CommitResponse::commit_stats will be set, but ReadWriteTransaction::end and Client::read_write_transaction_with_option only returns the commit timestamp.

I'd be happy to contribute a change to expose the commit stats in the Client and/or ReadWriteTransaction APIs, but naively changing end or read_write_transaction_with_option to also return commit stats would seem to be a breaking change.

Do you have any thoughts on the best way to expose the commit stats? Would a breaking change be acceptable or should we introduce new functions to be backwards compatible, e.g. end_with_stats and read_write_transaction_with_stats?