public Builder newBuilder() {
        return new Builder();
    }
@Test
    public void testDoubleBuild() {
        FetchSessionHandler handler = new FetchSessionHandler(LOG_CONTEXT, 1);
        FetchSessionHandler.Builder builder = handler.newBuilder();
        builder.add(new TopicPartition("foo", 0),
            new FetchRequest.PartitionData(Uuid.randomUuid(), 0, 100, 200, Optional.empty()));
        builder.build();
        assertThrows(Throwable.class, builder::build, "Expected calling build twice to fail.");
    }