Variable Field Names

I can accomplish this task by having seperate tick scripts, but since the scripts are doing the same thing but on different fields of a measurement, I figured I would combine them into one TICK script.

The issue I am having is two fold.

1 )AWS CW metrics sometimes will push out metrics that do not have the all fields populated. i.e. one point may have a tag for "domain_name" but the field names vary all over the place, so I needed to filter out only the points that have a "domain_name" and "cluster_status.yellow_maximum" or "cluster_status.red_maximum" So I am doing a eval/keep to filter that out. This works fine as long as I don't use a variable for the field, i.e.
```
|eval(lambda: "domain_name", lambda: "cluster_status.yellow_maximum")
.as('_domain_name', '_cluster_status')
.keep()
.quiet()
```
So problem #1 solved.

2) Now the variable issue if I want to have a single TICK script the above would be:
```
|eval(lambda: "domain_name", lambda: measurement_field)
.as('_domain_name', '_cluster_status')
.keep()
.quiet()
```
where measurement_field is a variable, a string to be exact.
```
var measurement_field = "cluster_status.yellow_maximum"
```
Now this means that the metrics push into the next node would the proper domain name, but simply the string value of "measurement_field"
```
[elasticsearch_status-yellow:log5] 2017/01/23 20:33:48 I! {"Name":"cloudwatch_aws_es","Database":"local","RetentionPolicy":"default","Group":"","Dimensions":{"ByName":false,"TagNames":null},"Tags":{"deployment":"local","domain_name":"test"},"Fields":{"_cluster_status":"cluster_status.yellow_maximum","_domain_name":"test","cluster_status.yellow_maximum":1},"Time":"2017-01-23T20:33:48.51813888Z"}
```

So my question is in a lambda expression, how do I go about expanding the variable variable? Kinda
like a double eval??

I am running kapacitor 1.1.1

Any help would be greatly appreciated.

No repository file, test, or entry point is named. Start by reproducing the TICK 1.1.1 eval/keep example with a literal field and with measurement_field; done means the variable selects the intended measurement field rather than emitting the variable name as a value.

Written by the indexing model from the issue text.