[SPARK-34366][SQL] Add interface for DS v2 metrics

### What changes were proposed in this pull request?

This patch proposes to add a few public API change to DS v2, to make DS v2 scan can report metrics to Spark.

Two public interfaces are added.

* `CustomMetric`: metric interface at the driver side. It basically defines how Spark aggregates task metrics with the same metric name.
* `CustomTaskMetric`: task metric reported at executors. It includes a name and long value. Spark will collect these metric values and update internal metrics.

There are two public methods added to existing public interfaces. They are optional to DS v2 implementations.

* `PartitionReader.currentMetricsValues()`: returns an array of CustomTaskMetric. Here is where the actual metrics values are collected. Empty array by default.
* `Scan.supportedCustomMetrics()`: returns an array of supported custom metrics `CustomMetric`. Empty array by default.

### Why are the changes needed?

In order to report custom metrics, we need some public API change in DS v2 to make it possible.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

This only adds interfaces. In follow-up PRs where adding implementation there will be tests added. See #31451 and #31398 for some details and manual test there.

Closes #31476 from viirya/SPARK-34366.

Authored-by: Liang-Chi Hsieh <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

sql/catalyst/src/main/java/org/apache/spark/sql/connector/CustomMetric.java

@@ -0,0 +1,51 @@
+/*
+ * 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.spark.sql.connector;
+
+import org.apache.spark.annotation.Evolving;
+
+/**
+ * A custom metric. Data source can define supported custom metrics using this interface.
+ * During query execution, Spark will collect the task metrics using {@link CustomTaskMetric}
+ * and combine the metrics at the driver side. How to combine task metrics is defined by the
+ * metric class with the same metric name.
+ *
+ * @since 3.2.0
+ */
+@Evolving
+public interface CustomMetric {
+    /**
+     * Returns the name of custom metric.
+     */
+    String name();
+
+    /**
+     * Returns the description of custom metric.
+     */
+    String description();
+
+    /**
+     * The initial value of this metric.
+     */
+    long initialValue = 0L;
+
+    /**
+     * Given an array of task metric values, returns aggregated final metric value.
+     */
+    String aggregateTaskMetrics(long[] taskMetrics);
+}

sql/catalyst/src/main/java/org/apache/spark/sql/connector/CustomTaskMetric.java

@@ -0,0 +1,46 @@
+/*
+ * 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.spark.sql.connector;
+
+import org.apache.spark.annotation.Evolving;
+import org.apache.spark.sql.connector.read.PartitionReader;
+
+/**
+ * A custom task metric. This is a logical representation of a metric reported by data sources
+ * at the executor side. During query execution, Spark will collect the task metrics per partition
+ * by {@link PartitionReader} and update internal metrics based on collected metric values.
+ * For streaming query, Spark will collect and combine metrics for a final result per micro batch.
+ *
+ * The metrics will be gathered during query execution back to the driver and then combined. How
+ * the task metrics are combined is defined by corresponding {@link CustomMetric} with same metric
+ * name. The final result will be shown up in the data source scan operator in Spark UI.
+ *
+ * @since 3.2.0
+ */
+@Evolving
+public interface CustomTaskMetric {
+    /**
+     * Returns the name of custom task metric.
+     */
+    String name();
+
+    /**
+     * Returns the long value of custom task metric.
+     */
+    long value();
+}

sql/catalyst/src/main/java/org/apache/spark/sql/connector/read/PartitionReader.java

@@ -21,6 +21,7 @@
 import java.io.IOException;
 
 import org.apache.spark.annotation.Evolving;
+import org.apache.spark.sql.connector.CustomTaskMetric;
 
 /**
  * A partition reader returned by {@link PartitionReaderFactory#createReader(InputPartition)} or
@@ -48,4 +49,12 @@ public interface PartitionReader<T> extends Closeable {
    * Return the current record. This method should return same value until `next` is called.
    */
   T get();
+
+  /**
+   * Returns an array of custom task metrics. By default it returns empty array.
+   */
+  default CustomTaskMetric[] currentMetricsValues() {
+    CustomTaskMetric[] NO_METRICS = {};
+    return NO_METRICS;
+  }
 }

sql/catalyst/src/main/java/org/apache/spark/sql/connector/read/Scan.java

@@ -18,6 +18,7 @@
 package org.apache.spark.sql.connector.read;
 
 import org.apache.spark.annotation.Evolving;
+import org.apache.spark.sql.connector.CustomMetric;
 import org.apache.spark.sql.connector.read.streaming.ContinuousStream;
 import org.apache.spark.sql.connector.read.streaming.MicroBatchStream;
 import org.apache.spark.sql.types.StructType;
@@ -102,4 +103,13 @@ default MicroBatchStream toMicroBatchStream(String checkpointLocation) {
   default ContinuousStream toContinuousStream(String checkpointLocation) {
     throw new UnsupportedOperationException(description() + ": Continuous scan are not supported");
   }
+
+  /**
+   * Returns an array of supported custom metrics with name and description.
+   * By default it returns empty array.
+   */
+  default CustomMetric[] supportedCustomMetrics() {
+    CustomMetric[] NO_METRICS = {};
+    return NO_METRICS;
+  }
 }