Add monitoring on the proxy server

[teolemon]

What

The server proxy was stopped this week-end, leading to query.openfoodfacts.org timeouting, leading to the whole website timeouting.

Tasks

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1. Add monitoring of NGINX reverse proxies openfoodfacts-monitoring#96
   monitoring +1
2. We should ensure that workers are freed within a reasonable amount of time (like 10s, since the answer time has been greatly reduced by openfoodfacts-query). This ensures that workers are not depleted in case openfoodfacts-query runs slow or fails alltogether
3. We could even have an automatic fallback on the product opener classic tags should openfoodfacts-query fail.