Provide chart function labels over the wire instead of making them up

Summary: Ref T13279. Makes charts incrementally more useful by allowing the server to provide labels and colors for functions.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: yelirekim

Maniphest Tasks: T13279

Differential Revision: https://secure.phabricator.com/D20501