docs: public void addSubMetricQuery(final String metric_query, 
                                  final int sub_query_index,
                                  final int param_index) {
      if (metric_query == null || metric_query.isEmpty()) {
        throw new IllegalArgumentException("Metric query cannot be null or empty");
      }
      if (sub_query_index < 0) {
        throw new IllegalArgumentException("Sub query index must be 0 or greater");
      }
      if (param_index < 0) {
        throw new IllegalArgumentException("Parameter index must be 0 or greater");
      }
      if (sub_metric_queries == null) {
        sub_metric_queries = Maps.newHashMap();
      }
      sub_metric_queries.put(sub_query_index, metric_query);
      parameter_index.put(param_index, Parameter.METRIC_QUERY);
    }