nixos/opentelemetry-collector: Use SupplementaryGroups to add the dynamic user to systemd-journal group

OpenTelemetry collector's Journald Receiver has not enough permissions

nixos/modules/services/monitoring/opentelemetry-collector.nix

@@ -65,6 +65,10 @@ in
           NoNewPrivileges = true;
           WorkingDirectory = "/var/lib/opentelemetry-collector";
           StateDirectory = "opentelemetry-collector";
+          SupplementaryGroups = [
+            # allow to read the systemd journal for opentelemetry-collector
+            "systemd-journal"
+          ];
         };
     };
   };