Instrument BlockingThreadPoolExecutor.

service/src/main/java/org/whispersystems/textsecuregcm/push/PushSender.java

@@ -56,7 +56,7 @@ public class PushSender implements Managed {
     this.apnSender          = apnSender;
     this.webSocketSender    = websocketSender;
     this.queueSize          = queueSize;
-    this.executor           = new BlockingThreadPoolExecutor(50, queueSize);
+    this.executor           = new BlockingThreadPoolExecutor("pushSender", 50, queueSize);
 
     SharedMetricRegistries.getOrCreate(Constants.METRICS_NAME)
                           .register(name(PushSender.class, "send_queue_depth"),

service/src/main/java/org/whispersystems/textsecuregcm/util/BlockingThreadPoolExecutor.java

@@ -1,22 +1,36 @@
 package org.whispersystems.textsecuregcm.util;
 
+import com.codahale.metrics.MetricRegistry;
+import com.codahale.metrics.SharedMetricRegistries;
+import com.codahale.metrics.Timer;
+
 import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.Semaphore;
 import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
 
+import static com.codahale.metrics.MetricRegistry.name;
+
 public class BlockingThreadPoolExecutor extends ThreadPoolExecutor {
 
   private final Semaphore semaphore;
+  private final Timer     acquirePermitTimer;
 
-  public BlockingThreadPoolExecutor(int threads, int bound) {
+  public BlockingThreadPoolExecutor(String name, int threads, int bound) {
     super(threads, threads, 1, TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>());
     this.semaphore = new Semaphore(bound);
+
+    final MetricRegistry metricRegistry = SharedMetricRegistries.getOrCreate(Constants.METRICS_NAME);
+
+    this.acquirePermitTimer = metricRegistry.timer(name(getClass(), name, "acquirePermit"));
+    metricRegistry.gauge(name(getClass(), name, "permitsAvailable"), () -> semaphore::availablePermits);
   }
 
   @Override
   public void execute(Runnable task) {
-    semaphore.acquireUninterruptibly();
+    try (final Timer.Context ignored = acquirePermitTimer.time()) {
+      semaphore.acquireUninterruptibly();
+    }
 
     try {
       super.execute(task);

service/src/test/java/org/whispersystems/textsecuregcm/tests/util/BlockingThreadPoolExecutorTest.java

@@ -10,7 +10,7 @@ public class BlockingThreadPoolExecutorTest {
 
   @Test
   public void testBlocking() {
-    BlockingThreadPoolExecutor executor = new BlockingThreadPoolExecutor(1, 3);
+    BlockingThreadPoolExecutor executor = new BlockingThreadPoolExecutor("test", 1, 3);
     long                       start    = System.currentTimeMillis();
 
     executor.execute(new Runnable() {