code: public Observable<ExtensionInner> getAsync(String resourceGroupName, String clusterName, String extensionName) {
          return getWithServiceResponseAsync(resourceGroupName, clusterName, extensionName).map(new Func1<ServiceResponse<ExtensionInner>, ExtensionInner>() {
              @Override
              public ExtensionInner call(ServiceResponse<ExtensionInner> response) {
                  return response.body();
              }
          });
      }
docs: Gets the extension properties for the specified HDInsight cluster extension.
  
  @param resourceGroupName The name of the resource group.
  @param clusterName The name of the cluster.
  @param extensionName The name of the cluster extension.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the ExtensionInner object