garage01: fix Caddy metrics deprecation warning

Use handle directive instead of path in site address for the metrics
endpoint, as the latter is deprecated in Caddy 2.10.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

services/garage/default.nix

@@ -53,8 +53,11 @@
         reverse_proxy http://localhost:3900
       }
 
-      http://garage01.home.2rjus.net:9117/metrics {
-        metrics
+      http://garage01.home.2rjus.net:9117 {
+        handle /metrics {
+          metrics
+        }
+        respond 404
       }
     '';
   };