private void reportMissingRegionServers(final ClusterMetrics clusterMetrics) {
    final Set<Address> regionServers = collectKnownRegionServers(clusterMetrics);
    final Set<Address> missingRegionServers = new HashSet<>(initialRegionServers);
    missingRegionServers.removeAll(regionServers);
    if (!missingRegionServers.isEmpty()) {
      final StringBuilder stringBuilder =
        new StringBuilder().append("region server(s) are missing from this cluster report");
      missingRegionServers.stream().sorted()
        .forEach(address -> stringBuilder.append("\n  ").append(address));
      getLogger().warn(stringBuilder.toString());
    }
  }
Build a set of all the host:port pairs of region servers known to this cluster.
hbase-it/src/test/java/org/apache/hadoop/hbase/chaos/actions/DumpClusterStatusAction.java