No way to tell if StackdriverExporter::Register was successful

[arfaian]

Ideally, the Register method for the StackdriverExporter would return a Status object. Looking at the code, this may not be trivial to implement since the registration is actually handled in a separate thread. Even so, it would still be useful to allow client code to block until registration is complete (either successful or unsuccessful) and take some action based on the result.