Add aggregate functions to query builder

[asimpson]

In order to support additional alerting functionality (e.g. fire an alert when the number of issues crosses an interval) we should support aggregate functionality in the query builder.

[vanchaxy]

Isn't this already covered by reduce expression?

E.g: this alert will fire when you have more than 300 issues in 3 hours

[asimpson]

@vanchaxy You are correct that reduce is the right expression but if you try a Issue query it won't work.

[louiswebdev]

We were also not able to create an alert for an issue query - but thanks for showing a possible alternative here
#73 (comment)

[kesik87]

Having same issue with creating grafana alert. Creating one for "stats" query type doesn't work for me as there is no filtering by environment.