default CompletableFuture<List<ServerName>> listUnknownServers() {
    return this.getClusterMetrics(EnumSet.of(Option.UNKNOWN_SERVERS))
      .thenApply(ClusterMetrics::getUnknownServerNames);
  }
List all the unknown region servers.
hbase-client/src/main/java/org/apache/hadoop/hbase/client/AsyncAdmin.java