Best effort shutdown of opamp client (signalfx#2738)

* best effort shutdown of opamp client

* spotless

Author: breedx-splk

custom/src/main/java/com/splunk/opentelemetry/opamp/OpampActivator.java

@@ -33,6 +33,7 @@
 import io.opentelemetry.sdk.autoconfigure.AutoConfiguredOpenTelemetrySdk;
 import io.opentelemetry.sdk.autoconfigure.spi.ConfigProperties;
 import io.opentelemetry.sdk.resources.Resource;
+import java.io.IOException;
 import java.time.Duration;
 import java.util.List;
 import java.util.logging.Logger;
@@ -60,28 +61,39 @@ public void afterAgent(AutoConfiguredOpenTelemetrySdk autoConfiguredOpenTelemetr
             OP_AMP_POLLING_INTERVAL, DEFAULT_DELAY_BETWEEN_REQUESTS.getNextDelay().toMillis());
 
     String endpoint = config.getString(OP_AMP_ENDPOINT);
-    startOpampClient(
-        endpoint,
-        resource,
-        pollingDuration,
-        new OpampClient.Callbacks() {
-          @Override
-          public void onConnect(OpampClient opampClient) {}
+    OpampClient client =
+        startOpampClient(
+            endpoint,
+            resource,
+            pollingDuration,
+            new OpampClient.Callbacks() {
+              @Override
+              public void onConnect(OpampClient opampClient) {}
 
-          @Override
-          public void onConnectFailed(OpampClient opampClient, @Nullable Throwable throwable) {
-            logger.log(WARNING, "Connection to OpAMP server failed", throwable);
-          }
+              @Override
+              public void onConnectFailed(OpampClient opampClient, @Nullable Throwable throwable) {
+                logger.log(WARNING, "Connection to OpAMP server failed", throwable);
+              }
 
-          @Override
-          public void onErrorResponse(
-              OpampClient opampClient, ServerErrorResponse serverErrorResponse) {
-            logger.log(WARNING, "OpAMP server returned error " + serverErrorResponse);
-          }
+              @Override
+              public void onErrorResponse(
+                  OpampClient opampClient, ServerErrorResponse serverErrorResponse) {
+                logger.log(WARNING, "OpAMP server returned error " + serverErrorResponse);
+              }
 
-          @Override
-          public void onMessage(OpampClient opampClient, MessageData messageData) {}
-        });
+              @Override
+              public void onMessage(OpampClient opampClient, MessageData messageData) {}
+            });
+    Runtime.getRuntime()
+        .addShutdownHook(
+            new Thread(
+                () -> {
+                  try {
+                    client.close();
+                  } catch (IOException e) {
+                    logger.log(WARNING, "Error shutting down OpAMP client", e);
+                  }
+                }));
   }
 
   static OpampClient startOpampClient(