Global sum of latest help

[Vilhelm von Ehrenheim]

Hi all!
First of all great work on the 2.2.0 release! really excited to start using
it.

I have a problem with how I should construct a pipeline that should emit a
sum of latest values which I hope someone might have some ideas on how to
solve.

Here is what I have:

I have a stateful stream of events that contain updates to a long amonst
other things. These events looks something like this

```
{"id": "1", parent_id: "a", "timestamp": 1. "amount": 1}
{"id": "2", parent_id: "a", "timestamp": 2, "amount": 3}
{"id": "1", parent_id: "a", "timestamp": 3, "amount": 2}
```

I want to emit sums of the `amount` per `parent_id` but only using the
latest record per `id`. Here that would result in sums of 1, 4 and then 5.

To make it harder I need to do this in a global window with triggering
based on element count. I could maybe combine that w a processing time
trigger though. At least I need a global sum over all events.

I have tried to do this with Latest.perKey and Sum.perKey but as you
probably realize that will give some strange results as the downstream sum
will not discard elements that are replaced by newer updates in the latest
transform.

I also though I could write a custom CombineFn for this but I need to do it
for different keys which leaves me really confused.

Any help or pointers are greatly appreciated.

Thanks!
Vilhelm von Ehrenheim