static ZooKeeperRegistrationSpec serverSets() {
        return new ServerSetsRegistrationSpecBuilder().build();
    }
Returns the {@link ZooKeeperRegistrationSpec} that registers the {@link Server} using the format of
<a href="https://twitter.github.io/finagle/docs/com/twitter/serverset.html">Finagle ServerSets</a>.

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