docs: public String writeStringField() {
      final List<String> strs = Lists.newArrayList();
      if (sub_expressions != null) {
        for (ExpressionTree sub : sub_expressions) {
          strs.add(sub.toString());
        }
      }
  
      if (sub_metric_queries != null) {
        final String sub_metrics = clean(sub_metric_queries.values());
        if (sub_metrics != null && sub_metrics.length() > 0) {
          strs.add(sub_metrics);
        }
      }
  
      final String inner_expression = DOUBLE_COMMA_JOINER.join(strs);
      return expression.writeStringField(func_params, inner_expression);
    }