[9.0] [APM] Prevent getChildrenGroupedByParentId to include the parent in the children list (#214957) (#215153)

# Backport

This will backport the following commits from `main` to `9.0`:
- [[APM] Prevent `getChildrenGroupedByParentId` to include the parent in
the children list
(#214957)](#214957)

<!--- Backport version: 9.6.6 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

<!--BACKPORT [{"author":{"name":"Sergi
Romeu","email":"[email protected]"},"sourceCommit":{"committedDate":"2025-03-19T12:28:20Z","message":"[APM]
Prevent `getChildrenGroupedByParentId` to include the parent in the
children list (#214957)\n\n## Summary\n\nCloses #213074\n\nThis PR
prevents `getChildrenGroupedByParentId` to include the parent\nitem in
the children list, as this was causing some duplication.\n\n| Before |
After
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Prevent `getChildrenGroupedByParentId` to include the parent in the
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Prevent `getChildrenGroupedByParentId` to include the parent in the
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prevents `getChildrenGroupedByParentId` to include the parent\nitem in
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After
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Prevent `getChildrenGroupedByParentId` to include the parent in the
children list (#214957)\n\n## Summary\n\nCloses #213074\n\nThis PR
prevents `getChildrenGroupedByParentId` to include the parent\nitem in
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BACKPORT-->

Co-authored-by: Sergi Romeu <[email protected]>

Author: kibanamachine

x-pack/solutions/observability/plugins/apm/public/components/app/transaction_details/waterfall_with_summary/waterfall_container/waterfall/waterfall_helpers/waterfall_helpers.ts

@@ -339,8 +339,23 @@ function reparentSpans(waterfallItems: IWaterfallSpanOrTransaction[]) {
   });
 }
 
-const getChildrenGroupedByParentId = (waterfallItems: IWaterfallSpanOrTransaction[]) =>
-  groupBy(waterfallItems, (item) => item.parentId ?? ROOT_ID);
+const getChildrenGroupedByParentId = (waterfallItems: IWaterfallSpanOrTransaction[]) => {
+  const childrenGroups = groupBy(waterfallItems, (item) => item.parentId ?? ROOT_ID);
+
+  const childrenGroupedByParentId = Object.entries(childrenGroups).reduce(
+    (acc, [parentId, items]) => {
+      // we shouldn't include the parent item in the children list
+      const filteredItems = items.filter((item) => item.id !== parentId);
+      return {
+        ...acc,
+        [parentId]: filteredItems,
+      };
+    },
+    {}
+  );
+
+  return childrenGroupedByParentId;
+};
 
 const getEntryWaterfallTransaction = (
   entryTransactionId: string,
@@ -517,15 +532,18 @@ function getChildren({
   path,
   waterfall,
   waterfallItemId,
+  rootId,
 }: {
   waterfallItemId: string;
   waterfall: IWaterfall;
   path: {
     criticalPathSegmentsById: Dictionary<CriticalPathSegment[]>;
     showCriticalPath: boolean;
   };
+  rootId: string;
 }) {
   const children = waterfall.childrenByParentId[waterfallItemId] ?? [];
+
   return path.showCriticalPath
     ? children.filter((child) => path.criticalPathSegmentsById[child.id]?.length)
     : children;
@@ -551,7 +569,12 @@ function buildTree({
   for (let queueIndex = 0; queueIndex < queue.length; queueIndex++) {
     const node = queue[queueIndex];
 
-    const children = getChildren({ path, waterfall, waterfallItemId: node.item.id });
+    const children = getChildren({
+      path,
+      waterfall,
+      waterfallItemId: node.item.id,
+      rootId: root.item.id,
+    });
 
     // Set childrenToLoad for all nodes enqueued.
     // this allows lazy loading of child nodes