Add pre-searcher creation hook in Engine (elastic#131633)

This adds a notifier method that offers access to the source and scope
of a searcher that we're about to create. This is useful so implementers
can reset stats based on the source and scope combination (the existing
acquireSearcherSupplier only offers access to the scope)

Author: andreidan

server/src/main/java/org/elasticsearch/index/engine/Engine.java

@@ -953,6 +953,9 @@ public final SearcherSupplier acquireSearcherSupplier(Function<Searcher, Searche
         return acquireSearcherSupplier(wrapper, SearcherScope.EXTERNAL);
     }
 
+    // Called before a {@link Searcher} is created, to allow subclasses to perform any stats or logging operations.
+    protected void onSearcherCreation(String source, SearcherScope scope) {}
+
     /**
      * Acquires a point-in-time reader that can be used to create {@link Engine.Searcher}s on demand.
      */
@@ -971,6 +974,7 @@ public SearcherSupplier acquireSearcherSupplier(Function<Searcher, Searcher> wra
                 @Override
                 public Searcher acquireSearcherInternal(String source) {
                     assert assertSearcherIsWarmedUp(source, scope);
+                    onSearcherCreation(source, scope);
                     return new Searcher(
                         source,
                         acquire,
@@ -1023,6 +1027,7 @@ public Searcher acquireSearcher(String source, SearcherScope scope, Function<Sea
             SearcherSupplier reader = releasable = acquireSearcherSupplier(wrapper, scope);
             Searcher searcher = reader.acquireSearcher(source);
             releasable = null;
+            onSearcherCreation(source, scope);
             return new Searcher(
                 source,
                 searcher.getDirectoryReader(),