this.size is currently always 0

this.size is currently always 0

https://github.com/apache/wayang/blob/745fc5cb9a765796533ca673c8d331fef15b2e63/wayang-platforms/wayang-flink/src/main/java/org/apache/wayang/flink/channels/DataStreamChannel.java#L42

```java

/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

package org.apache.wayang.flink.channels;

import org.apache.flink.streaming.api.datastream.DataStream;
import org.apache.wayang.core.optimizer.OptimizationContext;
import org.apache.wayang.core.plan.executionplan.Channel;
import org.apache.wayang.core.plan.wayangplan.OutputSlot;
import org.apache.wayang.core.platform.AbstractChannelInstance;
import org.apache.wayang.core.platform.ChannelDescriptor;
import org.apache.wayang.core.platform.ChannelInstance;
import org.apache.wayang.core.platform.Executor;
import org.apache.wayang.flink.execution.FlinkExecutor;

import java.util.OptionalLong;

public class DataStreamChannel extends Channel {

/**
* {@link ChannelInstance} implementation for {@link DataStream}s.
*/
public class Instance extends AbstractChannelInstance {

private DataStream dataStream;

// TODO: this.size is currently always 0
private long size;

public Instance(final FlinkExecutor executor,
final OptimizationContext.OperatorContext producerOperatorContext,
final int producerOutputIndex) {
super(executor, producerOperatorContext, producerOutputIndex);
}

public void accept(final DataStream dataStream) {
this.dataStream = dataStream;
}

@SuppressWarnings("unchecked")
public DataStream provideDataStream() {
return (DataStream) this.dataStream;
}

@Override
public OptionalLong getMeasuredCardinality() {
return this.size == 0 ? super.getMeasuredCardinality() : OptionalLong.of(this.size);
}

@Override
public DataStreamChannel getChannel() {
return DataStreamChannel.this;
}

@Override
protected void doDispose() {
this.dataStream = null;
}
}

public static final ChannelDescriptor DESCRIPTOR = new ChannelDescriptor(
DataStreamChannel.class, true, false);

public static final ChannelDescriptor DESCRIPTOR_MANY = new ChannelDescriptor(
DataStreamChannel.class, true, false);

public DataStreamChannel(final ChannelDescriptor descriptor, final OutputSlot outputSlot) {
super(descriptor, outputSlot);
assert descriptor == DESCRIPTOR || descriptor == DESCRIPTOR_MANY;
this.markForInstrumentation();
}

private DataStreamChannel(final DataStreamChannel parent) {
super(parent);
}

@Override
public Channel copy() {
return new DataStreamChannel(this);
}

@Override
public Instance createInstance(final Executor executor,
final OptimizationContext.OperatorContext producerOperatorContext,
final int producerOutputIndex) {
return new Instance((FlinkExecutor) executor, producerOperatorContext, producerOutputIndex);
}
}

```

af1d17f15876d937e6bb0fdc1597d80f23536731

Start in wayang-platforms/wayang-flink/src/main/java/org/apache/wayang/flink/channels/DataStreamChannel.java, focusing on Instance.accept(), the size field, and getMeasuredCardinality(). Trace how the DataStream instance can provide its cardinality, then ensure the measured size is populated and reported instead of remaining zero.

Written by the indexing model from the issue text.