Support for sum entry that is already summed

[hauleth]

This may be useful for situations like measuring :inet.getstat/{1,2} values without doing computations back and forth. The value returned by this metric is already summed. Technically we could use last_value metric there, but that would mark value as incorrect type in exported format. That can be problematic for some dashboards or other services that want to provide user meaningful graph by parsing type.