SERVER-24375 describes how deduping can use unbounded memory. Since we're not planning to fix that immediately with a disk-aware data structure, we should at least track it. This will make diagnosing cases where SERVER-24375 much, much easier.

A few notes:

• Probably want a few different counters, maybe on per stage (including SBE stages)
• We can make the counter coarse grain, and only update it every K rows to avoid any performance loss. Our use for this is the general trend rather than the exact amount of memory used.