default CompletableFuture<Collection<ServerName>> getRegionServers() {
    return getClusterMetrics(EnumSet.of(Option.SERVERS_NAME))
      .thenApply(ClusterMetrics::getServersName);
  }
Returns current live region servers list wrapped by {@link CompletableFuture}
hbase-client/src/main/java/org/apache/hadoop/hbase/client/AsyncAdmin.java