code: public Observable<VirtualMachineExtensionInner> getAsync(String resourceGroupName, String vmName, String vmExtensionName) {
          return getWithServiceResponseAsync(resourceGroupName, vmName, vmExtensionName).map(new Func1<ServiceResponse<VirtualMachineExtensionInner>, VirtualMachineExtensionInner>() {
              @Override
              public VirtualMachineExtensionInner call(ServiceResponse<VirtualMachineExtensionInner> response) {
                  return response.body();
              }
          });
      }
docs: The operation to get the extension.
  
  @param resourceGroupName The name of the resource group.
  @param vmName The name of the virtual machine containing the extension.
  @param vmExtensionName The name of the virtual machine extension.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the VirtualMachineExtensionInner object