Adapt to API change of TestMetricMaker

The constructor was made private in
I07739b44d22c0d2d7cb257443551478c458d7daf, hence now use the static
getInstance() method to retrieve the only instance.

Change-Id: I5f315027c66161752e854b392b70be81744f6edd

src/test/java/com/googlesource/gerrit/plugins/healthcheck/HttpActiveWorkersCheckTest.java

diff --git a/src/test/java/com/googlesource/gerrit/plugins/healthcheck/HttpActiveWorkersCheckTest.java b/src/test/java/com/googlesource/gerrit/plugins/healthcheck/HttpActiveWorkersCheckTest.java
index 60c233a..1bfc2eb 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/healthcheck/HttpActiveWorkersCheckTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/healthcheck/HttpActiveWorkersCheckTest.java
@@ -118,7 +118,7 @@
         healtchCheckConfig,
         injector.getInstance(ThreadSettingsConfig.class),
         injector.getInstance(MetricRegistry.class),
-        new TestMetricMaker());
+        TestMetricMaker.getInstance());
   }
 
   private Injector testInjector(AbstractModule testModule) {