Fix ingest stats test bug. (#50653)

This test code fixes a serialization test bug:
https://gradle-enterprise.elastic.co/s/7x2ct6yywkw3o

Rarely stats for the same processor are generated and
the production code then sums up these stats. However
the test code wasn't summing up in that case,
which caused inconsistencies between the actual and expected results.

Closes #50507

Author: martijnvg

server/src/test/java/org/elasticsearch/action/admin/cluster/stats/ClusterStatsNodesTests.java

@@ -70,9 +70,23 @@ public void testNetworkTypesToXContent() throws Exception {
     public void testIngestStats() throws Exception {
         NodeStats nodeStats = randomValueOtherThanMany(n -> n.getIngestStats() == null, NodeStatsTests::createNodeStats);
         SortedMap<String, long[]> processorStats = new TreeMap<>();
-        nodeStats.getIngestStats().getProcessorStats().values().forEach(l -> l.forEach(s -> processorStats.put(s.getType(),
-            new long[] { s.getStats().getIngestCount(), s.getStats().getIngestFailedCount(),
-                s.getStats().getIngestCurrent(), s.getStats().getIngestTimeInMillis()})));
+        nodeStats.getIngestStats().getProcessorStats().values().forEach(stats -> {
+            stats.forEach(stat -> {
+                processorStats.compute(stat.getType(), (key, value) -> {
+                    if (value == null) {
+                        return new long[] { stat.getStats().getIngestCount(), stat.getStats().getIngestFailedCount(),
+                            stat.getStats().getIngestCurrent(), stat.getStats().getIngestTimeInMillis()};
+                    } else {
+                        value[0] += stat.getStats().getIngestCount();
+                        value[1] += stat.getStats().getIngestFailedCount();
+                        value[2] += stat.getStats().getIngestCurrent();
+                        value[3] += stat.getStats().getIngestTimeInMillis();
+                        return value;
+                    }
+                });
+            });
+        });
+
         ClusterStatsNodes.IngestStats stats = new ClusterStatsNodes.IngestStats(Collections.singletonList(nodeStats));
         assertThat(stats.pipelineCount, equalTo(nodeStats.getIngestStats().getProcessorStats().size()));
         String processorStatsString = "{";