Enhanced math operations in widget metrics

## What?

A feature request to the awesome Graylog team to enhance available math operations in widget matrics.
For example, allow the user to use the following as a metric in the widgets :

- sum(field1)/60
- sum(sum(field1), sum(field2))
- (sum(field1) * 60) + sum(field2)

## Why?

Example scenario: calculate total minutes in a widget, based on "hours" and "minutes" fields.