In addition to the existing by-clause syntax:
sum(<expression>) by (<labels>) [keeping_extra]
...this allows the following new syntax:
sum by (<labels>) [keeping_extra] (<expression>)
Both orderings may be used in a single expression. It is up to the users
to establish guidelines around their usage.
Change-Id: Iba10c9cc5fb6ac62edfcf246d281473e82467992