Returns the number of items that have been read before filtering.
If no filter is set, then the readCount will be the same as size. But if a filter is applied, it could be that items
are read, but are filtered out. So if you are trying to make another read based on the ReadResultSet then you should
increment the sequence by readCount and not by size. Otherwise you will be re-reading the same filtered messages.
the number of items read (including the filtered ones).