Add promise labels to PromiseProxy

https://github.com/tildeio/rsvp.js/commit/9875d9f76ecbeb320c1f4d8aedbd23a3e0af4e64 added instrumentation to promises.  This PR lets `Ember.PromiseProxyMixin` account for promise labels.

Also part of https://github.com/tildeio/ember-extension/pull/76.