@Override
  public void labels(final DoubleSupplier valueSupplier, final String... labelValues) {
    final var valueList = List.of(labelValues);
    if (labelledCallbackData.putIfAbsent(valueList, new CallbackData(valueSupplier, labelValues))
        != null) {
      throw new IllegalArgumentException(
          String.format("A collector has already been created for label values %s", valueList));
    }
  }
Map label values with the collector callback data
metrics/core/src/main/java/org/hyperledger/besu/metrics/prometheus/AbstractPrometheusSuppliedValueCollector.java