static ZooKeeperDiscoverySpec serverSets(Function<? super ServerSetsInstance, Endpoint> converter) {
        return new ServerSetsDiscoverySpec(requireNonNull(converter, "converter"));
    }
Returns a {@link ZooKeeperDiscoverySpec} that is compatible with
<a href="https://twitter.github.io/finagle/docs/com/twitter/serverset.html">Finagle ServerSets</a>.

@param converter the converter to convert a {@link ServerSetsInstance} to an {@link Endpoint}.
                 If you don't want to connect to the service, you can simply return
                 {@code null} in the converter.

@see ZooKeeperRegistrationSpec#builderForServerSets()
zookeeper3/src/main/java/com/linecorp/armeria/client/zookeeper/ZooKeeperDiscoverySpec.java