Normally, this is what's supposed to happen when you invoke `rails g
generate migration`:

1. `Rails.application.load_generators` is called here:
https://github.com/rails/rails/blob/v4.2.10/railties/lib/rails/commands/commands_tasks.rb#L129
2. This loads the local `generator_templates` path in
https://github.com/rails/rails/blob/v4.2.10/railties/lib/rails/generators.rb#L69.

However, when metrics are enabled, the application is eager loaded here
in the initializer. This short-circuits the `load_generators` and loads
`ActiveRecord::Generators::MigrationGenerator` automatically, so the
custom path is never loaded properly.

With this change, we skip the initializing of metrics and eager loading
if generators are in use.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52651