Avoid memory leak when configuring all job count metrics with false

src/main/java/org/jenkinsci/plugins/prometheus/JobCollector.java

@@ -88,15 +88,17 @@
                         !PrometheusConfiguration.get().isCountSuccessfulBuilds() &&
                         !PrometheusConfiguration.get().isCountUnstableBuilds();
 
+        BuildCompletionListener listener = BuildCompletionListener.getInstance();
+
         if (ignoreBuildMetrics) {
+            listener.unregister();
             return samples;
         }
 
         // Below metrics use labelNameArray which might include the optional labels
         // of "parameters" or "status"
         summary = factory.createRunCollector(CollectorType.BUILD_DURATION_SUMMARY, labelNameArray, null);
-        BuildCompletionListener listener = BuildCompletionListener.getInstance();
-
+
         // Counter manager acts as a DB to retrieve any counters that are already in memory instead of reinitializing
         // them with each iteration of collect.
         var manager = CounterManager.getManager();

src/main/java/org/jenkinsci/plugins/prometheus/collectors/builds/BuildCompletionListener.java

@@ -41,6 +41,17 @@ protected BuildCompletionListener(){
         lock = new ReentrantLock();
     }
 
+    @Override
+    public void unregister() {
+        super.unregister();
+        try {
+            lock.lock();
+            this.runStack.clear();
+        } finally {
+            lock.unlock();
+        }
+    }
+
     /*
      * Extension tells Jenkins to register this class as a RunListener and to use
      * this method in order to retrieve an instance of the class. It is a singleton,
@@ -111,4 +122,8 @@ public void close() {
             }
         };
     }
+
+    List<Run<?, ?>> getRunStack() {
+        return runStack;
+    }
 }

src/test/java/org/jenkinsci/plugins/prometheus/collectors/builds/BuildCompletionListenerTest.java

@@ -0,0 +1,30 @@
+package org.jenkinsci.plugins.prometheus.collectors.builds;
+
+import hudson.model.Run;
+import hudson.model.TaskListener;
+import org.junit.jupiter.api.Test;
+import org.jvnet.hudson.test.Issue;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.mockito.Mockito.mock;
+
+class BuildCompletionListenerTest {
+
+    @Test
+    @Issue("#643")
+    void unregisterClearsRunStack() {
+        Run<?,?> mock = mock(Run.class);
+        TaskListener taskListener = mock(TaskListener.class);
+
+        BuildCompletionListener sut = BuildCompletionListener.getInstance();
+
+        sut.onCompleted(mock, taskListener);
+
+        assertEquals(1, sut.getRunStack().size());
+
+        sut.unregister();
+
+        assertEquals(0, sut.getRunStack().size(), "Unregister should clear the list. Otherwise a memory leak can occur.");
+
+    }
+}