default CompletableFuture<Collection<ServerName>> getBackupMasters() {
    return getClusterMetrics(EnumSet.of(Option.BACKUP_MASTERS))
      .thenApply(ClusterMetrics::getBackupMasterNames);
  }
Returns current backup master list wrapped by {@link CompletableFuture}
hbase-client/src/main/java/org/apache/hadoop/hbase/client/AsyncAdmin.java