Improve metrics to hide duplicates
Yesterday, a new step (`split-duckcb-index`) was added and run over all the datasets. Here are the metrics:

<img width="771" alt="Capture d’écran 2023-06-29 à 09 46 31" src="https://github.com/huggingface/datasets-server/assets/1676121/5daf2798-c79b-4ff9-a192-03ba38d4b149">

We can see that many new cache entries have been filled and that many of them are errors. But looking into the details, we can see that most of them are copies of the error in a previous step.

It would be useful to show the errors related only to the new step.
Improve metrics to hide duplicates: Yesterday, a new step (`split-duckcb-index`) was added and run over all the datasets. Here are the metrics:

<img width="771" alt="Capture d’écran 2023-06-29 à 09 46 31" src="https://github.com/huggingface/datasets-server/assets/1676121/5daf2798-c79b-4ff9-a192-03ba38d4b149">

We can see that many new cache entries have been filled and that many of them are errors. But looking into the details, we can see that most of them are copies of the error in a previous step.

It would be useful to show the errors related only to the new step.