code: public Observable<FeatureResultInner> registerAsync(String resourceProviderNamespace, String featureName) {
          return registerWithServiceResponseAsync(resourceProviderNamespace, featureName).map(new Func1<ServiceResponse<FeatureResultInner>, FeatureResultInner>() {
              @Override
              public FeatureResultInner call(ServiceResponse<FeatureResultInner> response) {
                  return response.body();
              }
          });
      }
docs: Registers the preview feature for the subscription.
  
  @param resourceProviderNamespace The namespace of the resource provider.
  @param featureName The name of the feature to register.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the FeatureResultInner object