SERVER-128940 introduces the LogicalSizeTracker class and the concept of a LogicalSizeTracker::Snapshot.

   struct LogicalSizeTracker::Snapshot {
        long long logicalBytesHot{0};
        long long logicalBytesCold{0};
    };

This data needs to include a version, and should be formalized so PALI can parse correctly and be robust against field changes and additions in the future.