Fix the cache metrics
<img width="381" alt="Capture d’écran 2023-10-03 à 10 34 02" src="https://github.com/huggingface/datasets-server/assets/1676121/9ed30ffc-f44b-4f45-b673-160decfeed34">


1. They are fluctuating. Maybe due to terminated uvicorn workers (see https://github.com/huggingface/datasets-server/issues/889)? Or to how we aggregate the metrics in Grafana (we take the median of all reports: `quantile by(kind) (0.5, sum by (pid, kind) (responses_in_cache_total))`)
2. We should have about the same number of entries for each split-level step, config-level step, and dataset-level step, but that's not the case.
Fix the cache metrics: <img width="381" alt="Capture d’écran 2023-10-03 à 10 34 02" src="https://github.com/huggingface/datasets-server/assets/1676121/9ed30ffc-f44b-4f45-b673-160decfeed34">


1. They are fluctuating. Maybe due to terminated uvicorn workers (see https://github.com/huggingface/datasets-server/issues/889)? Or to how we aggregate the metrics in Grafana (we take the median of all reports: `quantile by(kind) (0.5, sum by (pid, kind) (responses_in_cache_total))`)
2. We should have about the same number of entries for each split-level step, config-level step, and dataset-level step, but that's not the case.