The parentheses were necessary in the actual solution since the action
uses a regex in the `replacement` option which defaults to `$1` -- so it
expects to output the first matched group!

service/prometheus.md

@@ -1535,7 +1535,7 @@ field:
       - source_labels: ['host']
         target_label: 'alias'
         action: 'replace'
-        regex: '.+'
+        regex: '(.+)'
       - regex: '^host$'
         action: 'labeldrop'
 ```