private MetricHostProvider metricHostProvider(Map<String, String> overriddenHosts) {
    return new OverriddenMetricsHostProvider(overriddenHosts, metricHostProvider, injector.getInstance(ConfigHelper.class));
  }
PropertyHelper/AbstractPropertyProvider expect map of maps,
that's why we wrap metrics into map
ambari-server/src/main/java/org/apache/ambari/server/controller/internal/StackDefinedPropertyProvider.java