Window#first cuts off fields after execution, no option to preserve field with keep()

Nobody has claimed this yet.

Hi all,

I'm pretty new to tick script and I'm trying to make a window of 30s and emits every 15s.

Here is a my code:
```
var source = stream
|from()
.measurement('measurement')
.where(lambda: isPresent("retxPacketsReceived") AND isPresent("packetsReceived"))
|groupBy('device')
|log()
|window()
.period(30s)
.every(10s)

var retxPercentage = source
|first('retxPacketsReceived')
.as('firstRetxPacketsReceived')
|last('retxPacketsReceived')
.as('lastRetxPacketsReceived')
|first('packetsReceived')
.as('firstPacketsReceived')
|last('packetsReceived')
.as('lastPacketsReceived')
|eval(lambda: int("lastRetxPacketsReceived") - int("firstRetxPacketsReceived"))
.as('retxPercentageValue')
|log()

retxPercentage
|alert()
.warn(lambda: "retxPercentageValue" >= RETX_PACKETS_PERCENTAGE_WARN)
.warnReset(lambda: "retxPercentageValue" < RETX_PACKETS_PERCENTAGE_WARN)
.message('')
.id('{{index .Tags "device"}})
.idTag(idTag)
.levelTag(levelTag)
.messageField(messageField)
.durationField(durationField)
```

Here are the code I insert in Influx:
```
insert measurement,device=101 retxPacketsReceived=100,packetsReceived=100
insert measurement,device=101 retxPacketsReceived=100,packetsReceived=200
insert measurement,device=101 retxPacketsReceived=100,packetsReceived=300
```

The error I see in logs:
```
ts=2020-01-09T21:42:28.535Z lvl=error msg="failed to realize reduce context from fields" service=kapacitor task_master=main task=argo-dev node=last48 err="field \"retxPacketsReceived\" missing from point"
```
If I remove the `last` and only call single time to `first` then I can make it work. It's like calling `first` removes the field from the window or something like that...
Any idea?

Reproduce the supplied Kapacitor task with its window, first, and last pipeline, then inspect the Window#first behavior and the keep() option mentioned in the issue. Done means the window preserves retxPacketsReceived so both first and last reductions run without the missing-field error.

Written by the indexing model from the issue text.