Omit the django-prometheus middleware if `METRICS_ENABLED` is false

[jeremystretch]

Proposed Changes

NetBox supports the export of Prometheus metrics via django-prometheus if configured to do so. Metrics export is disabled by default, however the middleware is still active. This issue proposes enabling the middleware only when METRICS_ENABLED is true.

Justification

We may be able to optimize performance and shrink our bug surface a bit by disabling this middleware when metrics are not in use.