Java Datastore客户端调用runAggregation抛出“Not implemented”异常

我尝试使用Datastore聚合查询的新count特性（相关提交已在DatastoreImpl中实现该方法），但调用runAggregation方法时仍抛出如下异常：

UnsupportedOperationException: Not implemented.

我的代码是参考官方示例及Javadoc编写的：

val selectAllQuery = Query.newEntityQueryBuilder().setKind(entityName).build()
val aggregationQuery: AggregationQuery =
    Query.newAggregationQueryBuilder().addAggregation(count().`as`("total_count")).over(selectAllQuery).build()

val aggregationResults =
    datastore.runAggregation(aggregationQuery).forEach {
        println(it)
    }

内容的提问来源于stack exchange，提问作者Guillermo