Async OpenTelemetry tracing in common/grpc_utils

[dbolduc]

Some tasks from Milestone 2 of an internal design doc. With these tasks completed, we can support tracing an asynchronous gRPC client call.

• Implement EndSpan(...) for future<> types.
• Use shared_ptr<grpc::ClientContext> basically everywhere that has unique_ptr<grpc::ClientContext>
• Use CallContext instead of OptionsSpan. These are mainly in AsyncGrpcOperations.
• Add traced async backoff; update (Rest)Async{Retry,Polling}Loop to use it
• Add SetAttribute; Update AsyncPollingLoop to set LRO name.