code: public long getTotalUnknownCategoricalLevelsSeen() {
      long total = 0;
      for (AtomicLong l : unknownCategoricalsPerColumn.values()) {
        total += l.get();
      }
      return total;
    }
docs: Counts and returns all previously unseen categorical variables across all columns.
  Results may vary when called during prediction phase.
  
  @return A sum of all previously unseen categoricals across all columns