public static CustomMetricProvider3 getInstance(Map<String, String> properties, Map<String, Metric> metrics) {
      if (null == instance) {
        instance = new CustomMetricProvider3();
        instance.providerProperties.putAll(properties);
      }
      return instance;
    }
Test singleton accessor.  Public since instantiated via reflection.
ambari-server/src/test/java/org/apache/ambari/server/controller/internal/StackDefinedPropertyProviderTest.java