authelia: Allow anonymous access to AlertManager

Sometimes, I want to be able to look at active alerts without logging
in.  This rule allows read-only access to the AlertManager UI and API.
Unfortunately, the user experience when attempting to create a new
Silence using the UI without first logging in is suboptimal, but I think
that's worth the trade-off.

authelia/configuration.yml

@@ -36,6 +36,16 @@ access_control:
     networks:
     - internal
     policy: bypass
+  - domain: metrics.pyrocufflink.blue
+    networks:
+    - internal
+    resources:
+    - '^/alertmanager([/?].*)?$'
+    methods:
+    - GET
+    - HEAD
+    - OPTIONS
+    policy: bypass
 
 authentication_backend:
   ldap: