SharedFiltererImpl::getApproximateSize() returns zero rather than the actual size. Looking at the code it seems very likely this is a typo that got missed during development / code review. It also appears there is a similar bug/typo in ShardKeyPattern::getApproximateSize().

The goal of this task is to look at ShardFiltererImpl::getApproximateSize() and ShardKeyPattern::getApproximateSize() and to fix the implementations of these methods to produce correct results.