Enable metrics for Dev, Staging, and Production by default

In order to better catch issues related to Prometheus and the gem, we need to ensure that the feature is enabled in Dev, Staging, and Production environments. In 10.2, we missed a critical bug in the gem which broke GitLab if someone had monitoring enabled.

To help reduce the chance of this happening again, the Prometheus Metrics setting needs to be enabled. If it does not run stable here, we cannot release.