Remove usage of parallel stream for feature value map generation (#751)

Co-authored-by: Khor Shu Heng <khor.heng@gojek.com>

Author: khorshuheng

serving/src/main/java/feast/serving/service/OnlineServingService.java

@@ -98,7 +98,6 @@ public GetOnlineFeaturesResponse getOnlineFeatures(GetOnlineFeaturesRequest requ
           if (isStale(featureSetRequest, entityRow, featureRow)) {
             featureSetRequest
                 .getFeatureReferences()
-                .parallelStream()
                 .forEach(
                     ref -> {
                       populateStaleKeyCountMetrics(project, ref);