Adds `USE_NEW_LOAD_BALANCER_QUERY` ENV feature flag

This new flag allows to switch to the new query that can work in
patroni standby cluster, querying the leader as the "primary" equivalent

lib/gitlab/database.rb

@@ -259,18 +259,25 @@ module Gitlab
     # @param [ActiveRecord::Connection] ar_connection
     # @return [String]
     def self.get_write_location(ar_connection)
-      row = ar_connection
-              .select_all(<<~SQL)
-              SELECT CASE 
-                WHEN pg_is_in_recovery() = true AND EXISTS (SELECT 1 FROM pg_stat_get_wal_senders())
-                  THEN pg_last_wal_replay_lsn()::text 
-                WHEN pg_is_in_recovery() = false 
-                  THEN pg_current_wal_insert_lsn()::text
-                  ELSE NULL
-                END AS location;
+      use_new_load_balancer_query = Gitlab::Utils.to_boolean(ENV['USE_NEW_LOAD_BALANCER_QUERY'], default: false)
+
+      sql = if use_new_load_balancer_query
+              <<~NEWSQL
+                SELECT CASE
+                    WHEN pg_is_in_recovery() = true AND EXISTS (SELECT 1 FROM pg_stat_get_wal_senders())
+                      THEN pg_last_wal_replay_lsn()::text
+                    WHEN pg_is_in_recovery() = false
+                      THEN pg_current_wal_insert_lsn()::text
+                      ELSE NULL
+                    END AS location;
+              NEWSQL
+            else
+              <<~SQL
+                SELECT pg_current_wal_insert_lsn()::text AS location
               SQL
-              .first
+            end
 
+      row = ar_connection.select_all(sql).first
       row['location'] if row
     end