[ES|QL] Register AggregateMetricDoubleLiteral

This literal was not previously being registered because it was
primarily being used in testing code, but when aggregate_metric_double
is folded it is used as the java object representation, so it requires
registering.

Author: limotova

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/data/AggregateMetricDoubleBlockBuilder.java

@@ -206,6 +206,10 @@ public String getLabel() {
         }
     }
 
+    /**
+     * Literal to represent AggregateMetricDouble and primarily used for testing and during folding.
+     * For all other purposes it is preferred to use the builders over the literal for generating blocks when possible.
+     */
     public record AggregateMetricDoubleLiteral(Double min, Double max, Double sum, Integer count) implements GenericNamedWriteable {
         public AggregateMetricDoubleLiteral {
             min = (min == null || min.isNaN()) ? null : min;

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/ExpressionWritables.java

@@ -8,6 +8,8 @@
 package org.elasticsearch.xpack.esql.expression;
 
 import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
+import org.elasticsearch.compute.data.AggregateMetricDoubleBlockBuilder;
+import org.elasticsearch.plugins.SearchPlugin;
 import org.elasticsearch.xpack.esql.action.EsqlCapabilities;
 import org.elasticsearch.xpack.esql.core.expression.ExpressionCoreWritables;
 import org.elasticsearch.xpack.esql.expression.function.UnsupportedAttribute;
@@ -123,6 +125,12 @@ public static List<NamedWriteableRegistry.Entry> getNamedWriteables() {
         return entries;
     }
 
+    public static List<SearchPlugin.GenericNamedWriteableSpec> getGenericNamedWriteables() {
+        List<SearchPlugin.GenericNamedWriteableSpec> entries = new ArrayList<>();
+        entries.addAll(literals());
+        return entries;
+    }
+
     public static List<NamedWriteableRegistry.Entry> attributes() {
         List<NamedWriteableRegistry.Entry> entries = new ArrayList<>();
         entries.addAll(ExpressionCoreWritables.attributes());
@@ -265,4 +273,13 @@ private static List<NamedWriteableRegistry.Entry> fullText() {
     private static List<NamedWriteableRegistry.Entry> vector() {
         return VectorWritables.getNamedWritables();
     }
+
+    public static List<SearchPlugin.GenericNamedWriteableSpec> literals() {
+        return List.of(
+            new SearchPlugin.GenericNamedWriteableSpec(
+                "AggregateMetricDoubleLiteral",
+                AggregateMetricDoubleBlockBuilder.AggregateMetricDoubleLiteral::new
+            )
+        );
+    }
 }

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/plugin/EsqlPlugin.java

@@ -42,6 +42,7 @@
 import org.elasticsearch.plugins.ActionPlugin;
 import org.elasticsearch.plugins.ExtensiblePlugin;
 import org.elasticsearch.plugins.Plugin;
+import org.elasticsearch.plugins.SearchPlugin;
 import org.elasticsearch.rest.RestController;
 import org.elasticsearch.rest.RestHandler;
 import org.elasticsearch.threadpool.ExecutorBuilder;
@@ -90,7 +91,7 @@
 import java.util.function.Predicate;
 import java.util.function.Supplier;
 
-public class EsqlPlugin extends Plugin implements ActionPlugin, ExtensiblePlugin {
+public class EsqlPlugin extends Plugin implements ActionPlugin, ExtensiblePlugin, SearchPlugin {
     public static final boolean INLINE_STATS_FEATURE_FLAG = new FeatureFlag("esql_inline_stats").isEnabled();
 
     public static final String ESQL_WORKER_THREAD_POOL_NAME = "esql_worker";
@@ -371,4 +372,9 @@ public List<ExecutorBuilder<?>> getExecutorBuilders(Settings settings) {
     public void loadExtensions(ExtensionLoader loader) {
         extraCheckerProviders.addAll(loader.loadExtensions(PlanCheckerProvider.class));
     }
+
+    @Override
+    public List<SearchPlugin.GenericNamedWriteableSpec> getGenericNamedWriteables() {
+        return ExpressionWritables.getGenericNamedWriteables();
+    }
 }