modules/monitoring: add loki and promtail

modules/nixos/common-settings/mainland.nix

@@ -0,0 +1,38 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+let
+  inherit (lib)
+    mkIf
+    mkOption
+    types
+    mkDefault
+    ;
+
+  cfg = config.inMainland;
+in
+{
+  options.inMainland = mkOption {
+    type = types.bool;
+    default = config.time.timeZone == "Asia/Shanghai";
+  };
+
+  config = mkIf cfg {
+    nix.settings.extra-substituters = [
+      "https://mirrors.cernet.edu.cn/nix-channels/store?priority=20"
+    ];
+
+    networking.timeServers = [
+      "cn.ntp.org.cn"
+      "ntp.ntsc.ac.cn"
+    ];
+
+    services.dae = {
+      enable = mkDefault true;
+    };
+  };
+}