upgrading: 2.4.2 - Add @metric_defaults/proxy/metric

Author: sirainen

docs/installation/upgrade/2.4-to-2.4.x.md

@@ -36,6 +36,7 @@ changed to `2.4.2`.
 
 | Setting | Old Default Value | New Default Value |
 | ------- | ----------------- | ----------------- |
+| `@metric_defaults/proxy/metric` |  | Adds proxy_session_established |
 | `service/anvil/unix_listener/anvil/mode` | `0600` | `0660` |
 | `service/anvil/unix_listener/anvil/group` | (empty = root) | `$SET:default_internal_group` |
 | [[setting,mailbox_directory_name_legacy]] | `yes` | `no` |