Metrics with status UNDEF are not displayed in Status page

[pgiraud]

This then makes it impossible to show a metric and modify alerting for the given metric.

Steps to reproduce :

• navigate to /server/[{host}/{port}/alerting/cpu_core (either manually or by choosing "CPU Usage" on status page),
• edit the alert and uncheck the enabled checkbox,
• wait for the page to reload,
• then navigate to status page.

The "CPU usage" is misisng in the list and there's no way to enable the alert again unless we navigate manually to /alerting/cpu_core again.