Fix small inconsistency in doc_stats search source (#73372) (#73580)

This commit fixes a small inconsistency in the FrozenEngine
and Engine classes when they refer to the source used to
open a Searcher for loading doc stats.

Backport of #73372

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

@@ -101,6 +101,7 @@ public abstract class Engine implements Closeable {
     public static final String MAX_UNSAFE_AUTO_ID_TIMESTAMP_COMMIT_ID = "max_unsafe_auto_id_timestamp";
     public static final String SEARCH_SOURCE = "search"; // TODO: Make source of search enum?
     public static final String CAN_MATCH_SEARCH_SOURCE = "can_match";
+    protected static final String DOC_STATS_SOURCE = "doc_stats";
 
     protected final ShardId shardId;
     protected final Logger logger;
@@ -177,7 +178,7 @@ public DocsStats docStats() {
         // index.refresh_interval=-1 won't see any doc stats updates at all. This change will give more accurate statistics
         // when indexing but not refreshing in general. Yet, if a refresh happens the internal searcher is refresh as well so we are
         // safe here.
-        try (Searcher searcher = acquireSearcher("docStats", SearcherScope.INTERNAL)) {
+        try (Searcher searcher = acquireSearcher(DOC_STATS_SOURCE, SearcherScope.INTERNAL)) {
             return docsStats(searcher.getIndexReader());
         }
     }

x-pack/plugin/core/src/main/java/org/elasticsearch/index/engine/FrozenEngine.java

@@ -216,8 +216,8 @@ private Engine.Searcher openSearcher(String source, SearcherScope scope) throws
             case "load_seq_no":
             case "load_version":
                 assert false : "this is a read-only engine";
-            case "doc_stats":
-                assert false : "doc_stats are overwritten";
+            case DOC_STATS_SOURCE:
+                assert false : "doc stats are eagerly loaded";
             case "refresh_needed":
                 assert false : "refresh_needed is always false";
             case "segments":