When an accumulator operator is used with $group, the engine performs an accumulation process that processes each input value and updates/maintains "accumulator state" for each group. For each value processed, the current SBE implementation of the $mergeObjects operator will always update the accumulator state by allocating a new object rather than modifying the current existing object held in the accumulator state. This hurts the performance of the $mergeObjects operator in SBE.

The goal of this task is to improve the SBE implementation of $mergeObjects to avoid allocating a new object each time it processes a value.