Allow aggregate functions for dedup

Author: kanoshiou

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/aggregate/AggregateFunction.java

@@ -20,6 +20,7 @@
 import org.elasticsearch.xpack.esql.core.util.CollectionUtils;
 import org.elasticsearch.xpack.esql.io.stream.PlanStreamInput;
 import org.elasticsearch.xpack.esql.plan.logical.Aggregate;
+import org.elasticsearch.xpack.esql.plan.logical.Dedup;
 import org.elasticsearch.xpack.esql.plan.logical.LogicalPlan;
 
 import java.io.IOException;
@@ -139,7 +140,9 @@ public boolean equals(Object obj) {
     @Override
     public BiConsumer<LogicalPlan, Failures> postAnalysisPlanVerification() {
         return (p, failures) -> {
-            if (p instanceof Aggregate == false) {
+            // `dedup` for now is not exposed as a command,
+            // so allowing aggregate functions for dedup explicitly is just an internal implementation detail
+            if ((p instanceof Aggregate) == false && (p instanceof Dedup) == false) {
                 p.expressions().forEach(x -> x.forEachDown(AggregateFunction.class, af -> {
                     if (af instanceof Rate) {
                         failures.add(fail(af, "aggregate function [{}] not allowed outside METRICS command", af.sourceText()));