code: private Set<MachineMetric> customMachineMetrics() {
          Set<MachineMetric> customized = new HashSet<MachineMetric>();
          for (MetricType m: AwsSdkMetrics.getPredefinedMetrics()) {
              if (m instanceof MachineMetric)
                  customized.add((MachineMetric)m);
          }
          return customized;
      }
docs: Returns the set of custom machine metrics specified in the SDK metrics
  registry; or an empty set if there is none. Note any machine metrics
  found in the registry must have been custom specified, as the default
  behavior is to include all machine metrics when enabled.
  
  @return a non-null set of machine metrics. An empty set means no custom
  machine metrics have been specified.