Additional log on plug-in self-registration and failures.

Whenever a new class is self-registered from a plug-in
additional INFO message is displayed with class name
and listener / exported interface.

Additionally when a Listener fails to get created
from injection into a DynamicSet, an error is logged
allowing the plug-in developer to understand what
was the problem.

Without this patch, you may see your plus-in loading
correctly but then the listeners being discarded
silently and you would spend a lot of time figuring
out where the problem was.

Change-Id: Ief3d3c5a3012136f73296a3afa5c85c22fc38bb7

gerrit-extension-api/pom.xml

@@ -47,6 +47,11 @@ limitations under the License.
       <groupId>org.apache.tomcat</groupId>
       <artifactId>servlet-api</artifactId>
     </dependency>
+
+    <dependency>
+      <groupId>org.slf4j</groupId>
+      <artifactId>slf4j-api</artifactId>
+    </dependency>
   </dependencies>
 
   <build>

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/registration/DynamicSet.java

@@ -25,6 +25,9 @@ import com.google.inject.name.Named;
 import com.google.inject.util.Providers;
 import com.google.inject.util.Types;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.util.Collection;
 import java.util.Iterator;
 import java.util.NoSuchElementException;
@@ -39,6 +42,7 @@ import java.util.concurrent.atomic.AtomicReference;
  * registrations to decide between singleton and non-singleton members.
  */
 public class DynamicSet<T> implements Iterable<T> {
+  private static final Logger log = LoggerFactory.getLogger(DynamicSet.class);
   /**
    * Declare a singleton {@code DynamicSet<T>} with a binder.
    * <p>
@@ -147,7 +151,8 @@ public class DynamicSet<T> implements Iterable<T> {
             try {
               next = p.get();
             } catch (RuntimeException e) {
-              // TODO Log failed member of DynamicSet.
+              log.error("Cannot get iterated object from provider " + p
+                  + ": object discared", e);
             }
           }
         }

gerrit-server/src/main/java/com/google/gerrit/server/plugins/AutoRegisterModules.java

@@ -192,10 +192,12 @@ class AutoRegisterModules {
 
     if (is("org.apache.sshd.server.Command", clazz)) {
       if (sshGen != null) {
+        PluginLoader.log.info("Exporting " + clazz + " as SSH Command");
         sshGen.export(export, clazz);
       }
     } else if (is("javax.servlet.http.HttpServlet", clazz)) {
       if (httpGen != null) {
+        PluginLoader.log.info("Exporting " + clazz + " as HTTP Servlet");
         httpGen.export(export, clazz);
         listen(clazz, clazz);
       }
@@ -247,6 +249,7 @@ class AutoRegisterModules {
         TypeLiteral<?> tl = TypeLiteral.get(type);
         if (env.hasDynamicSet(tl)) {
           sysSingletons.add(clazz);
+          PluginLoader.log.info("Registering " + clazz + " to listener " + tl);
           sysListen.put(tl, clazz);
         } else if (env.hasDynamicMap(tl)) {
           if (clazz.getAnnotation(Export.class) == null) {
@@ -255,6 +258,7 @@ class AutoRegisterModules {
                 clazz.getName(), rawType.getName()));
           }
           sysSingletons.add(clazz);
+          PluginLoader.log.info("Registering " + clazz + " to listener " + tl);
           sysListen.put(tl, clazz);
         } else {
           throw new InvalidPluginException(String.format(