DB load balancing: Log when we use a different secondary host when there's a query conflict

We currently log that a host is offline in LoadBalancer#read

We should also log when we need to try a different host due to query conflicts (https://gitlab.com/gitlab-org/gitlab-ee/blob/4d73f53e7f5c5fdd1989ac2c71d51a6d40dbe3f0/ee/lib/gitlab/database/load_balancing/load_balancer.rb#L42-43)