@Override
    public void onCompletion(Long result, Exception exception) {
      callbackWrapper.updateBytesRead(result);
      if (exception != null || isLast) {
        callbackWrapper.invokeCallback(exception);
      }
    }
Updates the number of bytes read and invokes {@link ReadIntoCallbackWrapper#invokeCallback(Exception)} if
{@code exception} is not {@code null} or if this is the last piece of content in the request.
@param result The result of the request. This would be non null when the request executed successfully
@param exception The exception that was reported on execution of the request
ambry-test-utils/src/main/java/com/github/ambry/rest/MockRestRequest.java