public ProgressLoggers add(Supplier<String> obj) {
    loggers.add(new Object() {
      @Override
      public String toString() {
        return obj.get();
      }
    });
    return this;
  }
Adds the current number of items in a queue and the queue's size to the output.
planetiler-core/src/main/java/com/onthegomap/planetiler/stats/ProgressLoggers.java