public static SerializerFactory getFactory(ProcessingEnvironment processingEnv) {
    return new SerializerFactoryImpl(loadExtensions(processingEnv), processingEnv);
  }
Returns a {@link SerializerFactory} with {@link SerializerExtension} instances provided by the
{@link java.util.ServiceLoader}.
value/src/main/java/com/google/auto/value/extension/serializable/serializer/SerializerFactoryLoader.java