fix: add SERVICE_NAME constant to pubsub client (#8026)

Author: mrVrAlex

PubSub/src/PubSubClient.php

@@ -108,6 +108,9 @@ class PubSubClient
         SchemaServiceClient::class
     ];
 
+    // The name of the service. Used in debug logging.
+    private const SERVICE_NAME = 'google.pubsub.v2.Pubsub';
+
     /**
      * @var RequestHandler
      * @internal