Users of WiredTiger have a general desire to know how much memory WiredTiger is consuming outside of the cache. It might help us identify where memory usage is coming from if we tag allocations and frees into different categories (for cache, not for cache to start with).
We should carefully consider whether this would be useful. The WiredTiger cache tracking is complex and disassociated from the actual memory allocation, and involves adjustments for assumed allocator overhead. As such it's not likely that we could create a set of information that could be used for double-accounting in terms of comparing what WiredTiger reports with what the allocator reports.