nixos/cupsd: Introduce services.printing.logLevel option

Previously we indirectly suggested that the user use
services.printing.extraConf to set this, but this doesn't work with the
default merge ordering. Fix this by making it an independent option.
Fixes #39611.

nixos/modules/services/printing/cupsd.nix

@@ -83,6 +83,8 @@ let
 
     WebInterface ${if cfg.webInterface then "Yes" else "No"}
 
+    LogLevel ${cfg.logLevel}
+
     ${cfg.extraConf}
   '';
 
@@ -165,6 +167,15 @@ in
         '';
       };
 
+      logLevel = mkOption {
+        type = types.str;
+        default = "info";
+        example = "debug";
+        description = ''
+          Specifies the cupsd logging verbosity.
+        '';
+      };
+
       extraFilesConf = mkOption {
         type = types.lines;
         default = "";
@@ -180,7 +191,7 @@ in
         example =
           ''
             BrowsePoll cups.example.com
-            LogLevel debug
+            MaxCopies 42
           '';
         description = ''
           Extra contents of the configuration file of the CUPS daemon
@@ -345,8 +356,6 @@ in
 
     services.printing.extraConf =
       ''
-        LogLevel info
-
         DefaultAuthType Basic
 
         <Location />