CompositeSyntheticFieldLoader tweak (#128461) (#128474)

SourceLoader.SyntheticFieldLoader#write(...) is hot code, avoid using streams api and use loop and compute sum.

Author: martijnvg

server/src/main/java/org/elasticsearch/index/mapper/CompositeSyntheticFieldLoader.java

@@ -91,7 +91,10 @@ public boolean hasValue() {
 
     @Override
     public void write(XContentBuilder b) throws IOException {
-        var totalCount = parts.stream().mapToLong(Layer::valueCount).sum();
+        long totalCount = 0;
+        for (Layer part : parts) {
+            totalCount += part.valueCount();
+        }
 
         if (totalCount == 0) {
             return;