Improve service handling and add unit tests for threading behavior in MetricExporter

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=130790170

java/google/registry/monitoring/metrics/MetricExporter.java

@@ -14,13 +14,18 @@
 
 package google.registry.monitoring.metrics;
 
+import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
+
 import com.google.common.base.Optional;
 import com.google.common.collect.ImmutableList;
 import com.google.common.util.concurrent.AbstractExecutionThreadService;
+import java.io.IOException;
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.Executor;
+import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ThreadFactory;
+import java.util.logging.Level;
 import java.util.logging.Logger;
 
 /**
@@ -49,11 +54,18 @@ class MetricExporter extends AbstractExecutionThreadService {
     logger.info("Started up MetricExporter");
     while (isRunning()) {
       Optional<ImmutableList<MetricPoint<?>>> batch = writeQueue.take();
+      logger.info("Got a batch of points from the writeQueue");
       if (batch.isPresent()) {
-        for (MetricPoint<?> point : batch.get()) {
-          writer.write(point);
+        logger.info("Batch contains data, writing to MetricWriter");
+        try {
+          for (MetricPoint<?> point : batch.get()) {
+            writer.write(point);
+          }
+          writer.flush();
+        } catch (IOException exception) {
+          logger.log(
+              Level.SEVERE, "Threw an exception while writing or flushing metrics", exception);
         }
-        writer.flush();
       } else {
         logger.info("Received a poison pill, stopping now");
         // An absent optional indicates that the Reporter wants this service to shut down.
@@ -64,6 +76,21 @@ class MetricExporter extends AbstractExecutionThreadService {
 
   @Override
   protected Executor executor() {
-    return Executors.newSingleThreadExecutor(threadFactory);
+    final ExecutorService executor = Executors.newSingleThreadExecutor(threadFactory);
+    // Make sure the ExecutorService terminates when this service does.
+    addListener(
+        new Listener() {
+          @Override
+          public void terminated(State from) {
+            executor.shutdown();
+          }
+
+          @Override
+          public void failed(State from, Throwable failure) {
+            executor.shutdown();
+          }
+        },
+        directExecutor());
+    return executor;
   }
 }