Metrics - Prometheus doesn't work

Hi, I wanna to enable prometheus metrics for gitlab.

So I add gitlab_rails['env'] = { 'prometheus_multiproc_dir' => '/dev/shm' } into my gitlab.rb

And than select the checkbox, restart my gitlab like thie image:

But I still can't visit the metrics page through url:

http://git.foobar.com/-/metrics?token=cXaSVqabTgDFvJgwV6Uz

Response message list this:

# Metrics are disabled, see: http://git.foobar.com/help/administration/monitoring/prometheus/gitlab_metrics#gitlab-prometheus-metrics

Have I config something wrong?

Thank you very mach.