Limit the feature categories we initialize metrics with to the categories we use

Currently, we initialize the http_requests_total with all feature_categories in [Gitlab::Metrics::RequestsRackMiddleware.initialize_metrics`](https://gitlab.com/gitlab-org/gitlab/blob/75bea0e57263e1d4833704392b277809504da68d/lib/gitlab/metrics/requests_rack_middleware.rb#L53).

This causes unneeded cardinality in Prometheus, since not all of the categories defined in feature_categories.yml are used.

We should limit this to only the used feature categories. By checking in Prometheus for which categories we actually incremented the counter. We should then make sure that all controllers and API endpoints tagged with a category has it's category initialized in the every_controller_spec.rb and every_api_endpoint_spec.rb.