feat: add --debug flag for metrics troubleshooting

Add a --debug flag to the listener command that enables debug-level
logging. When enabled, the listener logs detailed information about
metrics recording including:

- When deployment start/end metrics are recorded
- The action, success status, and duration being recorded
- Whether metrics are enabled or disabled (skipped)

This helps troubleshoot issues where deployment metrics appear to
remain at zero after deployments.

Also add extraArgs option to the NixOS module to allow passing
additional arguments like --debug to the service.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nixos/module.nix

@@ -19,7 +19,8 @@ let
   ++ lib.optionals cfg.metrics.enable [
     "--metrics-enabled"
     "--metrics-addr ${lib.escapeShellArg cfg.metrics.address}"
-  ]);
+  ]
+  ++ cfg.extraArgs);
 
   # Extract port from metrics address for firewall rule
   metricsPort = let
@@ -122,6 +123,13 @@ in
         description = "Open firewall for metrics port";
       };
     };
+
+    extraArgs = lib.mkOption {
+      type = lib.types.listOf lib.types.str;
+      default = [ ];
+      description = "Extra command line arguments to pass to the listener";
+      example = [ "--debug" ];
+    };
   };
 
   config = lib.mkIf cfg.enable {