Fix aggregatorFactory meta merge exception (apache#7504)

extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/hll/HllSketchMergeAggregatorFactory.java

@@ -25,6 +25,8 @@
 import com.yahoo.sketches.hll.TgtHllType;
 import com.yahoo.sketches.hll.Union;
 import org.apache.druid.query.aggregation.Aggregator;
+import org.apache.druid.query.aggregation.AggregatorFactory;
+import org.apache.druid.query.aggregation.AggregatorFactoryNotMergeableException;
 import org.apache.druid.query.aggregation.AggregatorUtil;
 import org.apache.druid.query.aggregation.BufferAggregator;
 import org.apache.druid.segment.ColumnSelectorFactory;
@@ -51,6 +53,23 @@ public HllSketchMergeAggregatorFactory(
     super(name, fieldName, lgK, tgtHllType);
   }
 
+  @Override
+  public AggregatorFactory getMergingFactory(AggregatorFactory other) throws AggregatorFactoryNotMergeableException
+  {
+    if (other.getName().equals(this.getName()) && other instanceof HllSketchMergeAggregatorFactory) {
+      HllSketchMergeAggregatorFactory castedOther = (HllSketchMergeAggregatorFactory) other;
+
+      return new HllSketchMergeAggregatorFactory(
+              getName(),
+              getName(),
+              Math.max(getLgK(), castedOther.getLgK()),
+              getTgtHllType().compareTo(castedOther.getTgtHllType()) < 0 ? castedOther.getTgtHllType() : getTgtHllType()
+      );
+    } else {
+      throw new AggregatorFactoryNotMergeableException(this, other);
+    }
+  }
+
   @Override
   public String getTypeName()
   {