Produce metric for max thread pool size (#319)

Author: tommy-bo

simpleclient_jetty_jdk8/src/main/java/io/prometheus/client/jetty/QueuedThreadPoolStatisticsCollector.java

@@ -36,6 +36,8 @@ public List<MetricFamilySamples> collect() {
             QueuedThreadPool::getThreads),
         buildGauge("jetty_queued_thread_pool_threads_idle", "Number of idle threads",
             QueuedThreadPool::getIdleThreads),
+        buildGauge("jetty_queued_thread_pool_threads_max", "Max size of thread pool",
+            QueuedThreadPool::getMaxThreads),
         buildGauge("jetty_queued_thread_pool_jobs", "Number of total jobs",
             QueuedThreadPool::getQueueSize));
   }

simpleclient_jetty_jdk8/src/test/java/io/prometheus/client/jetty/QueuedThreadPoolStatisticsCollectorTest.java

@@ -50,6 +50,9 @@ public void metricsGathered() throws Exception {
     assertTrue(
         CollectorRegistry.defaultRegistry.getSampleValue("jetty_queued_thread_pool_threads_idle",
             LABEL_NAMES, labelValues) > 0);
+    assertTrue(
+        CollectorRegistry.defaultRegistry.getSampleValue("jetty_queued_thread_pool_threads_max",
+            LABEL_NAMES, labelValues) == 200);
     assertNotNull(CollectorRegistry.defaultRegistry.getSampleValue("jetty_queued_thread_pool_jobs",
         LABEL_NAMES, labelValues));
   }