Allow anonymous access to zot /metrics endpoint

Add accessControl.metrics.users with empty string to allow
unauthenticated Prometheus/Alloy scraping. Zot represents
anonymous users with an empty username internally.

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

ansible/roles/zot/templates/config.json.j2

@@ -26,6 +26,9 @@
       "apikey": true
     },
     "accessControl": {
+      "metrics": {
+        "users": [""]
+      },
       "repositories": {
         "**": {
           "policies": [