The SizeCountStore API includes two ways to write to the underlying container: SizeCountStore::write() and SizeCountStore::insert(). insert() asserts that the container does not contain a pre-existing entry for the provided key. write() upserts. 

Since write() is a void function, callers do not know if the write was an update or an insert. As a result, The insertCount and updateCount metrics cannot be updated.

These two member functions can be combined if we strengthen the return type. Combine SizeCountStore::write() and SizeCountStore::insert into simply SizeCountStore::update() (or SizeCountStore::write(), though there is enthusiasm for using "update" generically in the StorEx layer).

boost::optional<SizeCountStore::Entry> update(OperationContext* opCtx, UUID uuid, const Entry& entry);

Return the old element, if it exists. For callers that previously used SizeCountStore::insert(), simply assert that the return value is boost::none.

As part of this change, track insertCount and updateCount again.