fix cluster command processed metric (#663)

libs/cluster/Session/ClusterCommands.cs

@@ -178,6 +178,7 @@ private void ProcessClusterCommands(RespCommand command, out bool invalidParamet
                 RespCommand.CLUSTER_SLOTSTATE => NetworkClusterSlotState(out invalidParameters),
                 _ => throw new Exception($"Unexpected cluster subcommand: {command}")
             };
+            this.sessionMetrics?.incr_total_cluster_commands_processed();
         }
     }
 }