docs: private static void createInternal(
        @Nullable Credentials credentials,
        String projectId,
        Duration exportInterval,
        MonitoredResource monitoredResource,
        @Nullable String metricNamePrefix,
        Map<LabelKey, LabelValue> constantLabels)
        throws IOException {
      synchronized (monitor) {
        checkState(instance == null, "Stackdriver stats exporter is already created.");
        instance =
            new StackdriverStatsExporter(
                projectId,
                createMetricServiceClient(credentials),
                exportInterval,
                monitoredResource,
                metricNamePrefix,
                constantLabels);
      }
    }