feat: Initialize default no-op PubsubTracer in Publisher and Subscriber

google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/BasePubsubTracer.java

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2024 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.cloud.pubsub.v1;
+
+public class BasePubsubTracer implements PubsubTracer {
+  BasePubsubTracer() {}
+}

google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/Publisher.java

@@ -131,7 +131,7 @@ public class Publisher implements PublisherInterface {
 
   private final boolean enableOpenTelemetryTracing;
   private final OpenTelemetry openTelemetry;
-  private PubsubTracer tracer = null;
+  private PubsubTracer tracer = new BasePubsubTracer();
 
   /** The maximum number of messages in one request. Defined by the API. */
   public static long getApiMaxRequestElementCount() {

google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/Subscriber.java

@@ -151,7 +151,7 @@ public class Subscriber extends AbstractApiService implements SubscriberInterfac
 
   private final boolean enableOpenTelemetryTracing;
   private final OpenTelemetry openTelemetry;
-  private PubsubTracer tracer = null;
+  private PubsubTracer tracer = new BasePubsubTracer();
 
   private Subscriber(Builder builder) {
     receiver = builder.receiver;