DLB: Create new .com alerts based on the new application-side metrics

Context

Related to Container Registry: DLB Phase 1 (&14105 - closed).

Task

Once DLB: Update Grafana dashboards with new metrics (#1379 - closed) is complete, we should create alerts for GitLab.com based on the new Prometheus metrics so that we're alerted in case load balancing is not operating properly.