Add a shutdown monitor that publishes shutdown state as a metric

service/src/main/java/org/whispersystems/textsecuregcm/metrics/ApplicationShutdownMonitor.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2013-2021 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.metrics;
+
+import io.dropwizard.lifecycle.Managed;
+import io.micrometer.core.instrument.Metrics;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static com.codahale.metrics.MetricRegistry.name;
+
+/**
+ * A managed monitor that reports whether the application is shutting down as a metric. That metric can then be used in
+ * conjunction with other indicators to conditionally fire or suppress alerts.
+ */
+public class ApplicationShutdownMonitor implements Managed {
+
+  private final AtomicBoolean shuttingDown = new AtomicBoolean(false);
+
+  public ApplicationShutdownMonitor() {
+    Metrics.gauge(name(getClass().getSimpleName(), "shuttingDown"), shuttingDown, b -> b.get() ? 1 : 0);
+  }
+
+  @Override
+  public void start() throws Exception {
+    shuttingDown.set(false);
+  }
+
+  @Override
+  public void stop() throws Exception {
+    shuttingDown.set(true);
+  }
+}