Standardize access to metrics and healthcheck

In some apps, the metrics and healthcheck are public:

- https://datasets-server.huggingface.co/admin/metrics
- https://datasets-server.huggingface.co/sse/metrics
- https://datasets-server.huggingface.co/sse/healthcheck
- https://datasets-server.huggingface.co/healthcheck
- On others, it’s forbidden or not found:

- https://datasets-server.huggingface.co/metrics
- https://datasets-server.huggingface.co/filter/metrics

As @severo suggests, it should be coherent among all the services. (Do we want the metrics to be public, or not?)

Review the listed metrics and healthcheck endpoints across the services, including the /admin, /sse, /filter, and root paths. First clarify whether these endpoints should be public; done means the chosen access policy is applied consistently across all services and the endpoint behavior no longer differs unexpectedly.

Written by the indexing model from the issue text.