Exclude /metrics from LOGIN_REQUIRED

[larsweiler]

Environment

• Python version: 3.7.3
• NetBox version: 2.6-beta1

Proposed Functionality

With Prometheus Metrics from #3104 enabled and also LOGIN_REQUIRED enabled, the URL /metrics will redirect to the login page. This merge request will exclude /metrics from LOGIN_REQUIRED in the same way as /api is excluded.

Use Case

For monitoring with Prometheus a user login on a webpage is not feasible. Prometheus should access the metrics without login.

Database Changes

None.

External Dependencies

None.

[lampwins]

Closed in #3124