docs: public void validate() {
      if (interval == null || interval.isEmpty()) {
        throw new IllegalArgumentException("Missing or empty interval");
      }
      DateTime.parseDuration(interval);
      
      if (aggregator == null || aggregator.isEmpty()) {
        throw new IllegalArgumentException("Missing or empty aggregator");
      }
      try {
        Aggregators.get(aggregator.toLowerCase());
      } catch (final NoSuchElementException e) {
        throw new IllegalArgumentException("Invalid aggregator");
      }
      
      if (fill_policy != null) {
        fill_policy.validate();
      }
    }