Close Stackdriver client after closing registry (#4358)

Do not close the client before `close` has finished as `close` publishes the metrics one last time. Close the client upon start if necessary so that multiple stop/start cycles would not leak clients.

Fixes gh-4353

---------

Co-authored-by: Dimo Velev <dimo.velev1_ext@payback.net>

implementations/micrometer-registry-stackdriver/src/main/java/io/micrometer/stackdriver/StackdriverMeterRegistry.java

@@ -125,6 +125,7 @@ public void start(ThreadFactory threadFactory) {
                 logger.error("unable to start stackdriver, service settings are not available");
             }
             else {
+                shutdownClientIfNecessary(true);
                 try {
                     this.client = MetricServiceClient.create(metricServiceSettings);
                     super.start(threadFactory);
@@ -138,11 +139,57 @@ public void start(ThreadFactory threadFactory) {
 
     @Override
     public void stop() {
-        if (client != null)
-            client.shutdownNow();
         super.stop();
     }
 
+    @Override
+    public void close() {
+        try {
+            super.close();
+        }
+        finally {
+            shutdownClientIfNecessary(false);
+        }
+    }
+
+    protected void shutdownClientIfNecessary(final boolean quietly) {
+        if (client != null) {
+            if (!client.isShutdown()) {
+                try {
+                    client.shutdownNow();
+                    final boolean terminated = client.awaitTermination(10, TimeUnit.SECONDS);
+                    if (!terminated) {
+                        logger.warn("The metric service client failed to terminate within the timeout");
+                    }
+                }
+                catch (final RuntimeException e) {
+                    if (quietly) {
+                        logger.warn("Failed to shutdown the metric service client", e);
+                    }
+                    else {
+                        throw e;
+                    }
+                }
+                catch (final InterruptedException e) {
+                    Thread.currentThread().interrupt();
+                    return;
+                }
+            }
+            try {
+                client.close();
+            }
+            catch (final RuntimeException e) {
+                if (quietly) {
+                    logger.warn("Failed to close metric service client", e);
+                }
+                else {
+                    throw e;
+                }
+            }
+            client = null;
+        }
+    }
+
     @Override
     protected void publish() {
         if (client == null) {