@Override
  public synchronized void labels(
      final Supplier<ExternalSummary> summarySupplier, final String... labelValues) {
    final var valueList = List.of(labelValues);
    if (labelledCallbackData.containsKey(valueList)) {
      throw new IllegalArgumentException(
          String.format("A collector has already been created for label values %s", valueList));
    }

    labelledCallbackData.put(valueList, new CallbackData(summarySupplier, labelValues));
  }
Map label values with the collector callback data
metrics/core/src/main/java/org/hyperledger/besu/metrics/prometheus/PrometheusSuppliedSummary.java