Switch charts to Prometheus API endpoint CE-2

What does this MR do?

This is first part of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27849

• Add feature flag to frontend
• if enabled, fetch Dashboard endpoint. Show chart titles and info, but no query data

To enable feature:

echo 'Feature.enable(:environment_metrics_use_prometheus_endpoint)' | bundle exec rails c

Doesn't fetch Prometheus data, so renders empty charts:

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/58516