Add "jvm_objects_pending_finalization" counter providing access to MemoryMXBean.getObjectPendingFinalizationCount()

Author: rainerjung

simpleclient_hotspot/src/main/java/io/prometheus/client/hotspot/MemoryPoolsExports.java

@@ -48,6 +48,12 @@ void addMemoryAreaMetrics(List<MetricFamilySamples> sampleFamilies) {
     MemoryUsage heapUsage = memoryBean.getHeapMemoryUsage();
     MemoryUsage nonHeapUsage = memoryBean.getNonHeapMemoryUsage();
 
+    GaugeMetricFamily finalizer = new GaugeMetricFamily(
+        "jvm_objects_pending_finalization",
+        "The number of objects waiting in the finalizer queue.",
+        memoryBean.getObjectPendingFinalizationCount());
+    sampleFamilies.add(finalizer);
+
     GaugeMetricFamily used = new GaugeMetricFamily(
         "jvm_memory_bytes_used",
         "Used bytes of a given JVM memory area.",

simpleclient_hotspot/src/test/java/io/prometheus/client/hotspot/MemoryPoolsExportsTest.java

@@ -35,6 +35,7 @@ public void setUp() {
     when(mockPoolsBean2.getName()).thenReturn("PS Old Gen");
     when(mockPoolsBean2.getUsage()).thenReturn(mockUsage2);
     when(mockPoolsBean2.getCollectionUsage()).thenReturn(mockCollectionUsage2);
+    when(mockMemoryBean.getObjectPendingFinalizationCount()).thenReturn(10);
     when(mockMemoryBean.getHeapMemoryUsage()).thenReturn(mockUsage1);
     when(mockMemoryBean.getNonHeapMemoryUsage()).thenReturn(mockUsage2);
     when(mockUsage1.getUsed()).thenReturn(500000L);
@@ -174,6 +175,11 @@ public void testMemoryPools() {
 
   @Test
   public void testMemoryAreas() {
+    assertEquals(
+        10L,
+        registry.getSampleValue(
+            "jvm_objects_pending_finalization"),
+        .0000001);
     assertEquals(
         500000L,
         registry.getSampleValue(