update Group size only when groups size changes

Currently, we increment generic group metrics whenever we create a new Group object when we load a partition. This is incorrect as the partition may contain several records for the same group if in the active segment or if the segment has not yet been compacted. 

The same applies to removing groups; we can possibly have multiple group tombstone records. Instead, only increment the metric if we created a new group and only decrement the metric if the group exists.