Disable Prometheus metrics if initialization fails

Previously if the underlying filesystem ran of space, reads and writes
to mmap() regions would throw an ugly SIGBUS error and crash.

With prometheus-client-mmap v0.10.0,
`Prometheus::Client.reinitialize_on_pid_change` will now throw an
IOError if initialization fails for some reason. If this happens, we
disable internal Prometheus metrics to ensure the system stays up.

Closes https://gitlab.com/gitlab-org/gitlab/issues/24425