[GitHub] [spark] zsxwing commented on a change in pull request #31476: [SPARK-34366][SQL] Add interface for DS v2 metrics

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zsxwing commented on a change in pull request #31476:
URL: https://github.com/apache/spark/pull/31476#discussion_r570666630



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File path: sql/catalyst/src/main/java/org/apache/spark/sql/connector/CustomMetric.java
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@@ -0,0 +1,38 @@
+/*
+ * 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.

Review comment:
       Since this is a public API, it would be great to add more information to explain how to use APIs as a developer. For example,
   
   - If I define my own `CustomMetric`, how does Spark use it?
   - If I define a metric type that doesn't support `sum`, for example, measure the executor jvm heap size, how does Spark handle it?
   - If my `PartitionReaderWithMetrics` returns metrics for a partition, will Spark combine them for partitions of a Spark job?
   - In the streaming case, how does Spark combine metrics from different micro batches?
   - If I would like to report metrics in driver, how do I do it?
   
   I feel you might need to build an interface similar to the private `SQLMetric`.




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