Require PUSH_METRICS variable for metrics job

As PROMETHEUS_HOST and PUSHGATEWAY_URL are now defined at the project level rather than on the scheduled task, we need an additional variable check in order to prevent the metrics job being run outside of the schedule.

This is the cause of the recent Pipeline triggered but unable to find chatops job: https://ops.gitlab.net/gitlab-org/release/tools/-/pipelines/246661 response from ChatOps, because the metrics job was running for ChatOps pipelines, and we required those pipelines to only have a single chatops job.