Add a /metrics endpoint on the admin pod

[severo]

It will be useful:

1. to separate admin queries from the normal requests
2. to do only one request to the database (to get the cache and queue stats), since there is only 1 admin pod, against one request per pod on the api container

(maybe it would be a "datasets-server-prometheus-exporter")

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Maybe useful for us:

• https://github.com/huggingface/repository_scanner/blob/main/scan_api/src/infra/PromWorkerMetrics.ts
• https://github.com/huggingface/repository_scanner/blob/main/scan_api/src/infra/PromAPIMetrics.ts

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To do:

• create a starlette app and update DockerFile - see Add metrics endpoint to admin #340
• update image in infra and add the servicemonitor in infra - see Expose admin metrics #341