Add comment about journal internal labels

nixos/modules/monitoring/exporters/promtail.nix

@@ -39,6 +39,10 @@ in {
             host = hostName;
           };
         };
+        # The journal has many internal labels, that by default will
+        # be dropped because of their "__" prefix.  To keep them, rename them.
+        # https://grafana.com/docs/loki/latest/clients/promtail/scraping/#journal-scraping-linux-only
+        # https://www.freedesktop.org/software/systemd/man/systemd.journal-fields.html
         relabel_configs = [{
           source_labels = [ "__journal__systemd_unit" ];
           target_label = "unit";