Fix MQE top_n global query.

docs/en/api/metrics-query-expression.md

@@ -217,7 +217,15 @@ round(service_cpm / 60 , 2)
 The different operators could impact the `ExpressionResultType`, please refer to the above table.
 
 ## TopN Operation
-TopN Operation takes an expression and performs TopN calculation on its results.
+TopN Operation takes an expression and performs calculation to get the TopN of Services/Instances/Endpoints.
+The result depends on the `entity` condition in the query.
+- Global TopN: 
+  - The `entity` is empty.
+  - The result is the topN Services/Instances/Endpoints in the whole traffics. 
+  - **Notice**: If query the Endpoints metric, the global candidate set could be huge, please use it carefully. 
+- Service's Instances/Endpoints TopN: 
+  - The `serviceName` in the `entity` is not empty.
+  - The result is the topN Instances/Endpoints of the service.
 
 Expression:
 ```text
@@ -229,7 +237,8 @@ top_n(<metric_name>, <top_number>, <order>)
 `order` is the order of the top results. The value of `order` can be `asc` or `des`.
 
 For example:
-If we want to query the top 10 services with the highest `service_cpm` metric value, we can use the following expression:
+If we want to query the current service's top 10 instances with the highest `service_instance_cpm` metric value, we can use the following expression
+under specific service:
 
 ```text
 top_n(service_instance_cpm, 10, des)

docs/en/changes/changes.md

@@ -118,6 +118,7 @@
 * Support DoubleValue,IntValue,BoolValue in OTEL metrics attributes.
 * [Break Change] gGRPC metrics exporter unified the metric value type and support labeled metrics.
 * Add component definition(ID=152) for `c3p0`(JDBC3 Connection and Statement Pooling).
+* Fix MQE `top_n` global query. 
 
 #### UI
 

oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/query/AggregationQueryService.java

@@ -60,8 +60,11 @@ public List<SelectedRecord> sortMetrics(TopNCondition condition, Duration durati
         final String valueCName = ValueColumnMetadata.INSTANCE.getValueCName(condition.getName());
         List<KeyValue> additionalConditions = null;
         if (StringUtil.isNotEmpty(condition.getParentService())) {
+            if (condition.getNormal() == null) {
+                return Collections.emptyList();
+            }
             additionalConditions = new ArrayList<>(1);
-            final String serviceId = IDManager.ServiceID.buildId(condition.getParentService(), condition.isNormal());
+            final String serviceId = IDManager.ServiceID.buildId(condition.getParentService(), condition.getNormal());
             additionalConditions.add(new KeyValue(InstanceTraffic.SERVICE_ID, serviceId));
         }
         final List<SelectedRecord> selectedRecords = getAggregationQueryDAO().sortMetrics(

oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/query/input/RecordCondition.java

@@ -55,7 +55,7 @@ public RecordCondition(TopNCondition condition) {
             final Entity entity = new Entity();
             entity.setScope(condition.getScope() == null ? Scope.Service : condition.getScope());
             entity.setServiceName(condition.getParentService());
-            entity.setNormal(condition.isNormal());
+            entity.setNormal(condition.getNormal());
             this.parentEntity = entity;
         }
         this.topN = condition.getTopN();

oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/query/input/TopNCondition.java

@@ -46,7 +46,7 @@ public class TopNCondition {
      * Normal service is the service having installed agent or metrics reported directly. Unnormal service is
      * conjectural service, usually detected by the agent.
      */
-    private boolean normal;
+    private Boolean normal;
     /**
      * Indicate the metrics entity scope. Because this is a top list, don't need to set the Entity like the
      * MetricsCondition. Only accept scope = {@link Scope#Service} {@link Scope#ServiceInstance} and {@link