Fix issue #1249 and do not show or count private discussions in Tags summary.

extensions/tags/src/Listener/UpdateTagMetadata.php

@@ -114,6 +114,7 @@ class UpdateTagMetadata
             $tags = $discussion->tags;
         }
 
+        if (! $discussion->is_private) {
             foreach ($tags as $tag) {
                 $tag->discussions_count += $delta;
 
@@ -126,4 +127,5 @@ class UpdateTagMetadata
                 $tag->save();
             }
         }
+    }
 }