Respect $range in variable evaluation requests

[zarbis]

Unlike InfluxQL, Flux allows extracting tag values for specific time range. Right now it's impossible to utilize this feature in Grafana since $range in request like this:

from(bucket: "analytics")
|> range($range)
|> filter(fn: (r) =>  r._measurement == "foo" )
|> keyValues(keyColumns: ["bar"])

is always expanded into hard-coded start: -1h.

It leads to confusing and hard-to-pinpoint inconsistencies. For example:

1. you have table with daily summary of events by their type
2. time picker is set to Last 7 days
3. but sums of events are calculated only on types of events that happened only in last hour

This happens due to event types are retrieved from Grafana variable, populated by Flux query. This PR makes Grafana to template correct time range into variable query.

[zarbis]

@davkal PTAL

[jtlisi]

LGTM, I will test this tomorrow and try to get it merged.

[zarbis]

@jtlisi any progress on this?

[zarbis]

@jtlisi humble bump

[zarbis]

Anyone?

[zarbis]

@davkal PTAL

[zarbis]

@jtlisi Still working on this?

[davkal]

I think we need @briangann

[andig]

Lgtm. What is missing to get this merged?

/cc @daniellee

[zarbis]

@daniellee any chance for it to get merged within this year? :)

[ryantxu]

@zarbis -- I am going to try to release this in ~1 week. If there is anything else you are looking at I will have to time to focus on it next week

[zarbis]

@ryantxu any progress on this?

[srclosson]

Merged into master. Thanks!