Review fixes for dropwizard:dropwizard-testing (open-telemetry#17318)

Co-authored-by: otelbot <197425009+otelbot@users.noreply.github.com>

Author: otelbot[bot]

instrumentation/dropwizard/dropwizard-testing/src/test/java/io/opentelemetry/javaagent/instrumentation/dropwizard/DropwizardAsyncTest.java

@@ -23,6 +23,7 @@
 import java.net.URISyntaxException;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import javax.annotation.PreDestroy;
 import javax.ws.rs.GET;
 import javax.ws.rs.HeaderParam;
 import javax.ws.rs.Path;
@@ -63,7 +64,12 @@ public void run(Configuration configuration, Environment environment) {
 
   @Path("/")
   public static class AsyncServiceResource {
-    final ExecutorService executor = Executors.newSingleThreadExecutor();
+    private final ExecutorService executor = Executors.newSingleThreadExecutor();
+
+    @PreDestroy
+    public void stop() {
+      executor.shutdown();
+    }
 
     @GET
     @Path("success")