* Remove IndexedAttribute from api/label
IndexedAttribute is a synonym for IndexedLabel and a hold over from when
this iterator pattern had multiple implementations. Given it no longer
is uses, remove it to avoid introducing confusion to users.
* Update Changelog
* Update Changelog PR number
* New label set API
* Checkpoint
* Remove label.Labels interface
* Fix trace
* Remove label storage
* Restore metric_test.go
* Tidy tests
* More comments
* More comments
* Same changes as 654
* Checkpoint
* Fix batch labels
* Avoid Resource.Attributes() where possible
* Update comments and restore order in resource.go
* From feedback
* From feedback
* Move iterator_test & feedback
* Strenghten the label.Set test
* Feedback on typos
* Fix the set test per @krnowak
* Nit