code: @Nullable
    public static String getStickinessMetadataKeyFromServiceConfig(
        Map<String, ?> serviceConfig) {
      if (!serviceConfig.containsKey(SERVICE_CONFIG_STICKINESS_METADATA_KEY)) {
        return null;
      }
      return getString(serviceConfig, SERVICE_CONFIG_STICKINESS_METADATA_KEY);
    }
docs: Extracts the stickiness metadata key from a service config, or {@code null}.