* Split metric inst config
Instead of having the same configuration for both the Synchronous and
Asynchronous instruments, use specific options for both.
* Use Async/Sync opt for appropriate inst
* Update noop inst providers
* Update internal global impl
* Update sdk
* Remove unused method for callbackOption
* Test instrument configuration
* Lint imports
* Add changes to changelog
* Refactor callbacks and further split opts
Define callbacks to return the value observed. Because of the different
types returned for different observables, the callbacks and options are
move to the sync/async packages.
* Update noop impl
* Fix example_test.go
* Fix internal impl
* Update Callbacks
Return observations for distinct attr sets.
* Refactor common code in sdk/metric inst provider
* Update examples and prom exporter
* Generalize callback
* Update changelog
* Add unit tests for callback
* Add meter tests for cbacks on creation
* Rename Observations to Measurements
* Update Callback to accept an Observer
* Update SDK impl
* Move conf to instrument pkg
* Apply suggestions from code review
