chore: fix prepare for shutdown

Change-Id: I3068cfb508ff424215af4169edcc30a70b217a76

Author: igorbernstein2

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/internal/csm/exporter/BigtableCloudMonitoringExporter.java

@@ -140,6 +140,7 @@ public BigtableCloudMonitoringExporter(
     this.state = new AtomicReference<>(State.Running);
   }
 
+  @Override
   public void close() {
     client.close();
   }

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/internal/csm/exporter/BigtablePeriodicReader.java

@@ -64,12 +64,12 @@ public CompletableResultCode forceFlush() {
 
   @Override
   public CompletableResultCode shutdown() {
+    exporter.prepareForShutdown();
     return delegate.shutdown();
   }
 
   @Override
   public void close() throws IOException {
-    exporter.prepareForShutdown();
     delegate.close();
   }