nixos/grafana-agent: replace `settings.prometheus` with `settings.metrics`
nixos/grafana-agent: replace `settings.prometheus` with `settings.metrics`

According to https://github.com/grafana/agent/pull/1540, -prometheus.*
flages were deprecated in 0.19.0 in favor of the -metrics.*
counterparts. Same applies to `loki` being renamed to `logs`.

I'm not sure if the config file format is still supported (it could be),
but we shouldn't use deprecated configs.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs