Add extension point to register JGit PostUploadHooks

Plugins may register PostUploadHook instances in order to get notified
after JGit is done uploading a pack.

Now that PostUploadHook can be registered using the DynamicSet, use that
mechanism to register UploadPackMetricsHook.

Change-Id: If848bddc85ca8923a5d55f7dd448ac218c910e1c

gerrit-server/src/main/java/com/google/gerrit/server/git/GitModule.java

@@ -15,6 +15,9 @@
 package com.google.gerrit.server.git;
 
 import com.google.gerrit.extensions.config.FactoryModule;
+import com.google.gerrit.extensions.registration.DynamicSet;
+
+import org.eclipse.jgit.transport.PostUploadHook;
 
 /** Configures the Git support. */
 public class GitModule extends FactoryModule {
@@ -24,5 +27,7 @@ public class GitModule extends FactoryModule {
     factory(MetaDataUpdate.InternalFactory.class);
     bind(MetaDataUpdate.Server.class);
     bind(ReceiveConfig.class);
+    DynamicSet.bind(binder(), PostUploadHook.class)
+        .to(UploadPackMetricsHook.class);
   }
 }