Global variables like `$__interval` do not get parsed correctly

[danyonedwards]

What happened:
When using the Sentry Datasource, I am unable to use global variable from the query options in the query fields. E.g. using $__interval in the Interval field results in the error: "interval" should be in the format [number][unit] where unit is one of m/h/d/w

What you expected to happen:
I expect that I can use variables in the same way I can with other plugins

How to reproduce it (as minimally and precisely as possible):

1. Add Sentry as a Datasource
2. Set query type to Stats
3. Fill out required fields
4. In Interval field add variable $__interval

Screenshots

[aangelisc]

I can confirm we do not currently support this. I believe this is a feature request but we will discuss as a team.