Motivation for this change

noticed it was broken reviewing: #95516

  ERROR: Could not find a version that satisfies the requirement prometheus-flask-exporter (from mlflow==1.8.0) (from versions: none)
  ERROR: No matching distribution found for prometheus-flask-exporter (from mlflow==1.8.0)
  builder for '/nix/store/lk02jqf9zv1kijg0fi740b3y898ag2dp-python3.8-mlflow-1.8.0.drv' failed with exit code 1

prometheus-flask-exporter would need to be packaged for it to work again

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.