add serde for histo

extensions-core/histogram/src/main/java/org/apache/druid/query/aggregation/histogram/ApproximateHistogramDruidModule.java

@@ -68,5 +68,6 @@ public static void registerSerde()
   {
     ComplexMetrics.registerSerde(ApproximateHistogramFoldingSerde.TYPE_NAME, new ApproximateHistogramFoldingSerde());
     ComplexMetrics.registerSerde(FixedBucketsHistogramAggregator.TYPE_NAME, new FixedBucketsHistogramSerde());
+    ComplexMetrics.registerSerde(Histogram.TYPE.getComplexTypeName(), new HistogramSerde2());
   }
 }

extensions-core/histogram/src/main/java/org/apache/druid/query/aggregation/histogram/HistogramSerde2.java

@@ -0,0 +1,61 @@
+/*
+ * 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.druid.query.aggregation.histogram;
+
+import org.apache.druid.error.DruidException;
+import org.apache.druid.query.aggregation.BasicComplexMetricSerde;
+import org.apache.druid.segment.data.ObjectStrategy;
+import java.nio.ByteBuffer;
+
+public class HistogramSerde2 extends BasicComplexMetricSerde<Histogram>
+{
+  public HistogramSerde2()
+  {
+    super(Histogram.TYPE, new HistogramObjectStrategy());
+  }
+
+  static class HistogramObjectStrategy implements ObjectStrategy<Histogram>
+  {
+
+    @Override
+    public int compare(Histogram o1, Histogram o2)
+    {
+      return 0;
+    }
+
+    @Override
+    public Class<? extends Histogram> getClazz()
+    {
+      return Histogram.class;
+    }
+
+    @Override
+    public Histogram fromByteBuffer(ByteBuffer buffer, int numBytes)
+    {
+      throw DruidException.defensive("not supported");
+    }
+
+    @Override
+    public byte[] toBytes(Histogram val)
+    {
+      throw DruidException.defensive("not supported");
+    }
+  }
+}

extensions-core/histogram/src/test/java/org/apache/druid/query/aggregation/histogram/ApproximateHistogramAggregatorTest.java

@@ -38,6 +38,10 @@
 
 public class ApproximateHistogramAggregatorTest extends InitializedNullHandlingTest
 {
+  static {
+    ApproximateHistogramDruidModule.registerSerde();
+  }
+
   private void aggregateBuffer(TestFloatColumnSelector selector, BufferAggregator agg, ByteBuffer buf, int position)
   {
     agg.aggregate(buf, position);