We need to add cpp unitTest that stats added SERVER-57825 is being populated correctly while successful FCBIS is running across different stages:

• after backupCursor files is retrieved.
• after copying files from backupCursor
• after extended the backupCursor
• after copying files from extendedBackupCursor
• after completing the initialSync.

For reference: 

unitTest: initial_syncer_test.cpp   

Note that we could check also before backupCursor is opened but it is futile because there is no difference in stats to after backupCursor is opened.