We want to track aggregate operation stats inside SingleTransactionsStats using an OpDebug object that accumulates stats from all operations run inside a transaction. We should have an OpDebug member variable on SingleTransactionStats, and we should add to it the stats from the latest operation whenever that operation finishes i.e. whenever a transaction operation stashes its resources, we will add its CurOp::_debug field to the aggregate OpDebug instance stored in the SingleTransactionsStats instance on the session.