Update doc/user/project/integrations/prometheus.md

What does this MR do?

In gitlab-design#573 (closed), we decided to update the documentation as a first step in improving the experience of setting up the operations > metrics dashboards. This MR attempts to update the docs to match the flow actually needed to set up the Metrics Dashboard outlined as part of https://gitlab.com/gitlab-org/gitlab-ee/issues/11713