alert + influxDBOut with different names are not working

Nobody has claimed this yet.

I'm trying to make template script for metrics generation,but cannot use one value with different names (same for alert and influxDBOut)

```
var apiName = 'MobileApi'
var errorRateName = 'MobileApi_errorrate'
var resultDb = 'test'
var period = 5s

var total = stream
|from()
.database('metrics')
.retentionPolicy('autogen')
.measurement('metrics')
|default()
.tag('is-test', '')
|where(lambda: "is-test" != '1i')
|window()
.period(period)
.every(period)
.align()
|count(apiName)

var errors = stream
|from()
.database('metrics')
.retentionPolicy('autogen')
.measurement('metrics')
|default()
.tag('is-test', '')
.tag('is-error', '')
|where(lambda: "is-test" != '1i' AND "is-error" == 'True')
|window()
.period(period)
.every(period)
.align()
|count(apiName)

var errorRate = errors
|join(total)
.as('err', 'tot')
|eval(lambda: float("err.count") / float("tot.count") * 100.0)

var errorRateForDb = errorRate
.as(errorRateName)

var errorRateForAlert = errorRate
.as('errorrate_value')

// Alert
errorRateForAlert
|alert()
.id(errorRateName)
.crit(lambda: "errorrate_value" >= 50)
.critReset(lambda: "errorrate_value" < 50)
.warn(lambda: "errorrate_value" >= 10)
.warnReset(lambda: "errorrate_value" < 10)
.stateChangesOnly()
.message('{{ .ID }} value: {{index .Fields "errorrate_value"}}')
.slack()

errorRateForDb
|influxDBOut()
.database(resultDb)
.retentionPolicy('autogen')
.measurement('mainmetrics')

```

This script is working, but influxDBOut write value name as 'errorrate_value' not 'MobileApi'
this is strange, because 'errorrate_value' is set for errorRateForAlert and it is not linked with influxDBOut

If I don't use 'as' construction - all will work as expected, but I need to define 'MobileApi' also in
```
.crit(lambda: "errorrate_value" >= 50)
.critReset(lambda: "errorrate_value" < 50)
.warn(lambda: "errorrate_value" >= 10)
.warnReset(lambda: "errorrate_value" < 10)
```
block, and it will be not good template

Reproduce the supplied TICKscript and start by tracing how the `as()` calls affect the shared `errorRate` stream before it reaches `alert()` and `influxDBOut()`. Done means the alert can use `errorrate_value` while the database output preserves `MobileApi`; the payload names no files or tests to run.

Written by the indexing model from the issue text.