DropWizard metric support

Gerrit server supports defining and recording metrics.  Metric
reporters for monitoring can be implemented as plugins.  A basic
Graphite reporter is available here:

  https://gerrit-review.googlesource.com/#/c/72202/

Some example metrics are included in this change:

  change/query/query_latency
  (Query latency)

  sshd/sessions/connected
  (SSH sessions connected)

  sshd/sessions/created/count
  (SSH connections created)

  git/upload-pack
  (Upload packs requests)

Partially-by: Gustaf Lundh <gustaflh@axis.com>
Change-Id: I46a07aace57efe236ee724ec8d34c581e2c37965

gerrit-war/src/main/java/com/google/gerrit/httpd/WebAppInitializer.java

@@ -26,6 +26,7 @@ import com.google.gerrit.httpd.plugins.HttpPluginModule;
 import com.google.gerrit.lifecycle.LifecycleManager;
 import com.google.gerrit.lifecycle.LifecycleModule;
 import com.google.gerrit.lucene.LuceneIndexModule;
+import com.google.gerrit.metrics.dropwizard.DropWizardMetricMaker;
 import com.google.gerrit.reviewdb.client.AuthType;
 import com.google.gerrit.server.account.InternalAccountDirectory;
 import com.google.gerrit.server.cache.h2.DefaultCacheFactory;
@@ -288,6 +289,7 @@ public class WebAppInitializer extends GuiceServletContextListener
 
   private Injector createSysInjector() {
     final List<Module> modules = new ArrayList<>();
+    modules.add(new DropWizardMetricMaker.Module());
     modules.add(new WorkQueue.Module());
     modules.add(new ChangeHookRunner.Module());
     modules.add(new ReceiveCommitsExecutorModule());