Remove QueryCollectorContext abstraction (#95383)

The QueryCollectorContext abstraction was introduced by #24864 based on the requirement that the top docs collector creation needed to be delayed until after all the other collectors had been created. At the same time, collectors get wrapped depending on the search features enabled by the request, but the top score / total hit count collector is the root collector where the wrapping starts, which is why its corresponding context gets added at position 0 in the list of collector contexts.

Requirements have changed since #27666 , which means that we can go back to a simpler way of creating collectors and wrapping them. We no longer need a QueryCollectorContext abstraction, and we can instead create collectors straight-away, and wrap them as needed. This is much easier to follow compared to the very generic create(Collector) method that the context exposes.

TopDocsCollectorContext adds some value in that it incorporates all the logic around creating the top docs collector, yet it can be further simplified as well by making the postProcess method more specific.

Author: javanna

server/src/main/java/org/elasticsearch/search/profile/query/CollectorResult.java

@@ -67,7 +67,7 @@ public class CollectorResult implements ToXContentObject, Writeable {
     /**
      * A list of children collectors "embedded" inside this collector
      */
-    private List<CollectorResult> children;
+    private final List<CollectorResult> children;
 
     public CollectorResult(String collectorName, String reason, long time, List<CollectorResult> children) {
         this.collectorName = collectorName;

server/src/main/java/org/elasticsearch/search/profile/query/InternalProfileCollector.java

@@ -47,15 +47,15 @@ public class InternalProfileCollector implements Collector {
      */
     private final InternalProfileCollector[] children;
 
-    public InternalProfileCollector(Collector collector, String reason, InternalProfileCollector... children) {
+    public InternalProfileCollector(Collector collector, String reason, Collector... children) {
         this.collector = new ProfileCollector(collector);
         this.reason = reason;
         this.collectorName = deriveCollectorName(collector);
         Objects.requireNonNull(children, "children collectors cannot be null");
-        for (InternalProfileCollector child : children) {
-            Objects.requireNonNull(child, "child collector cannot be null");
+        this.children = new InternalProfileCollector[children.length];
+        for (int i = 0; i < children.length; i++) {
+            this.children[i] = (InternalProfileCollector) Objects.requireNonNull(children[i], "child collector cannot be null");
         }
-        this.children = children;
     }
 
     /**