Memtable heap size is severely underestimated

We seem to be estimating the size of the on-heap memtable metadata to be around half of what it actually is. For example, during a read benchmark which writes 1 million single-long rows the memtable reports

1000000 ops, 58.174MiB serialized bytes, 385.284MiB (19%) on heap, 0.000KiB (0%) off-heap

while a heap dump taken at this point:

lists an usage of about 666MB altogether.

Switching to offheap_objects, the reported numbers are

1000000 ops, 58.174MiB serialized bytes, 233.650MiB (12%) on heap, 53.406MiB (3%) off-heap

while actual heap usage:

is about 442MB.

Looking at the code we definitely are not counting the AtomicBTreePartition.Holder, EncodingStats, liveness and deletion info objects associated with each partition, and most probably others.