code: public MapReduceToCollectionOperation action(final String action) {
          notNull("action", action);
          isTrue("action must be one of: \"replace\", \"merge\", \"reduce\"", VALID_ACTIONS.contains(action));
          this.action = action;
          return this;
      }
docs: Sets the output action one of: "replace", "merge", "reduce"
  
  @param action the output action
  @return this
  @mongodb.driver.manual reference/command/mapReduce/#output-to-a-collection-with-an-action output with an action