Implementing multi-field "sum" function [FLINK-4029]

[Ivan Mushketyk]

Hi,

I would like to implement a multi-field "sum" function[1], but I am not
sure what would be the best way to approach it.
Should I add a new method to the DataStream class or would it be better to
add it to a specialized stream?
Should it be based on existing operators/streams, or should I implement a
new operator/stream?
What do you think about this?

[1] - https://issues.apache.org/jira/browse/FLINK-4029

Best regards,
Ivan.