Use read replica for count query

- This removes the incorrect clearing of the
global database load balancing session

ee/lib/analytics/group_activity_calculator.rb

@@ -15,14 +15,12 @@ module Analytics
     end
 
     def merge_requests_count
-      # Clear db load balancing session so that primary sticking
-      # does not apply in case a write has happened recently.
-      # We want to make sure the load of the following query
-      # does not land on the primary db.
-      ::Gitlab::Database::LoadBalancing::Session.clear_session
-
       @merge_requests_count ||=
-        Groups::RecentMergeRequestsCountService.new(@group, @current_user, params).count
+        # We want to make sure the load of the following query
+        # does not land on the read replica instead of the primary db
+        current_load_balancing_session.use_replicas_for_read_queries do
+          count_service.new(@group, @current_user, params).count
+        end
     end
 
     def new_members_count
@@ -44,5 +42,13 @@ module Analytics
         attempt_group_search_optimizations: true,
         attempt_project_search_optimizations: true }
     end
+
+    def current_load_balancing_session
+      ::Gitlab::Database::LoadBalancing::Session.current
+    end
+
+    def count_service
+      Groups::RecentMergeRequestsCountService
+    end
   end
 end