add service for workers
We need to access the /metrics endpoint in `servicemonitor`, and it surely requires a _service_ to _monitor_.

Note that we don't define a service "type", such as NodePort, because we don't want to expose the workers endpoints to the exterior.

It's a follow-up to https://github.com/huggingface/datasets-server/pull/2069
add service for workers: We need to access the /metrics endpoint in `servicemonitor`, and it surely requires a _service_ to _monitor_.

Note that we don't define a service "type", such as NodePort, because we don't want to expose the workers endpoints to the exterior.

It's a follow-up to https://github.com/huggingface/datasets-server/pull/2069