code: static boolean containsDisallowedLeLabelForHistogram(List<String> labelNames, Type type) {
      if (!Type.HISTOGRAM.equals(type)) {
        return false;
      }
      for (String label : labelNames) {
        if (LABEL_NAME_BUCKET_BOUND.equals(label)) {
          return true;
        }
      }
      return false;
    }
docs: https://github.com/prometheus/client_java/blob/af39ca948ca446757f14d8da618a72d18a46ef3d/simpleclient/src/main/java/io/prometheus/client/Histogram.java#L88