I am starting to migrate from legacy driver 1.0.6 to new c++11 driver 3.0.1.
In legacy driver, when we do bulk write, we were able to keep statistic information for the object to be updated. e.g:

loadCountInBytes_ += record.objsize();
bulk.find(BSON("EventId" << eventIDStr)).upsert().replaceOne(record);

where variable "record" is of type "mongo::BSONObj" (in our use case, it's an insert 99% of the time)
In new c++11 driver, what is the recommended way to track the above object size statistics (bytes loaded) since we are using the builders now (a filter builder and an update builder).
Thanks!
Judy