code: public DruidNodeDiscovery getForService(String serviceName)
    {
      return serviceDiscoveryMap.computeIfAbsent(
          serviceName,
          service -> {
  
            Set<NodeType> nodeTypesToWatch = DruidNodeDiscoveryProvider.SERVICE_TO_NODE_TYPES.get(service);
            if (nodeTypesToWatch == null) {
              throw new IAE("Unknown service [%s].", service);
            }
            ServiceDruidNodeDiscovery serviceDiscovery = new ServiceDruidNodeDiscovery(service, nodeTypesToWatch.size());
            DruidNodeDiscovery.Listener filteringGatheringUpstreamListener =
                serviceDiscovery.filteringUpstreamListener();
            for (NodeType nodeType : nodeTypesToWatch) {
              getForNodeType(nodeType).registerListener(filteringGatheringUpstreamListener);
            }
            return serviceDiscovery;
          }
      );
    }
docs: Get DruidNodeDiscovery instance to discover nodes that announce given service in its metadata.