Hi @hongalex and reviewers,

I'm currently reviewing this pull request and will post my feedback in a few minutes. In the meantime, here's a summary of the changes for you and other reviewers to quickly get up to speed:

This pull request, feat(pubsub): add v2 topic and publishing samples, aims to update the existing Google Cloud Pub/Sub samples to use the v2 version of the library. The author mentions that a previous PR moved the v1 samples, and this PR updates them in place. The description notes that subscription and schema handling will be addressed in separate PRs.

Here's a breakdown of the changes:

• pubsub/go.mod (lines 6-22, 48-66): Updated dependencies to include cloud.google.com/go/pubsub/v2 v2.0.0-00010101000000-000000000000 and other related libraries to their latest versions. A replace directive was added to point to a local copy of the v2 Pub/Sub library.
• pubsub/go.sum (lines 2-32, 95-226): Updated checksums for the updated dependencies.
• pubsub/topics/add_users.go (lines 18-67): Refactored the addUsers function to use the v2 Pub/Sub and IAM libraries. The IAM interaction is now explicitly using GetIamPolicy and SetIamPolicy calls, rather than the higher-level methods previously used.
• pubsub/topics/create.go (lines 20-43): Updated the create function to use the v2 Pub/Sub library and the new CreateTopic method.
• pubsub/topics/create_topic_gcs_ingestion.go (lines 21-81): Updated the createTopicWithCloudStorageIngestion function to use the v2 Pub/Sub library and the new CreateTopic method. The minimumObjectCreateTime parameter is now a timestamppb.Timestamp type, and a delimiter parameter was added.
• pubsub/topics/create_topic_kinesis_ingestion.go (lines 20-61): Updated the createTopicWithKinesisIngestion function to use the v2 Pub/Sub library and the new CreateTopic method.
• pubsub/topics/delete.go (lines 20-47): Updated the delete function to use the v2 Pub/Sub library and the new DeleteTopic method.
• pubsub/topics/list_subs.go (lines 18-55): Updated the listSubscriptions function to use the v2 Pub/Sub library and the new ListTopicSubscriptions method.
• pubsub/topics/list_topics.go (lines 18-55): Updated the listTopics function to use the v2 Pub/Sub library and the new ListTopics method.
• pubsub/topics/policy.go (lines 20-52): Updated the getIAMPolicy function to use the v2 Pub/Sub library and the new GetIamPolicy method. The output formatting was also changed.
• pubsub/topics/publish.go (lines 20-46): Updated the publish function to use the v2 Pub/Sub library and explicitly create a publisher. The publisher is reused across publishes.
• pubsub/topics/publish_custom.go (lines 20-46): Updated the publishCustomAttributes function to use the v2 Pub/Sub library and explicitly create a publisher. The publisher is reused across publishes.
• pubsub/topics/publish_flow_control.go (lines 23-63): Updated the publishWithFlowControlSettings function to use the v2 Pub/Sub library and explicitly create a publisher. The publisher is reused across publishes.
• pubsub/topics/publish_ordering.go (lines 22-77): Updated the publishWithOrderingKey function to use the v2 Pub/Sub library and explicitly create a publisher. The publisher is reused across publishes.
• pubsub/topics/publish_otel_tracing.go (lines 20-86): Updated the publishOpenTelemetryTracing function to use the v2 Pub/Sub library and explicitly create a publisher. The publisher is reused across publishes.
• pubsub/topics/publish_resume_ordering.go (lines 20-63): Updated the resumePublishWithOrderingKey function to use the v2 Pub/Sub library and explicitly create a publisher. The publisher is reused across publishes.
• pubsub/topics/publish_retry.go (lines 21-70): Updated the publishWithRetrySettings function to use the v2 Pub/Sub library and explicitly create a publisher. The publisher is reused across publishes. The PublisherCallOptions type was changed to TopicAdminCallOptions.
• pubsub/topics/publish_scale.go (lines 23-50): Updated the publishThatScales function to use the v2 Pub/Sub library and explicitly create a publisher. The publisher is reused across publishes.
• pubsub/topics/publish_settings.go (lines 22-57): Updated the publishWithSettings function to use the v2 Pub/Sub library and explicitly create a publisher. The publisher is reused across publishes.
• pubsub/topics/publish_single.go (lines 20-46): Updated the publishSingleGoroutine function to use the v2 Pub/Sub library and explicitly create a publisher. The publisher is reused across publishes.
• pubsub/topics/publish_with_compression.go (lines 20-48): Updated the publishWithCompression function to use the v2 Pub/Sub library and explicitly create a publisher. The publisher is reused across publishes.
• pubsub/topics/test_permissions.go (lines 20-53): Updated the testPermissions function to use the v2 Pub/Sub library and the new TestIamPermissions method.
• pubsub/topics/topics_test.go (lines 20-320): Updated several test functions to use the v2 Pub/Sub library and the new methods. The list function is now listTopics, and the policy function is now getIAMPolicy.
• pubsub/topics/update_topic_type.go (lines 20-63): Updated the updateTopicType function to use the v2 Pub/Sub library and the new UpdateTopic method. The updateCfg parameter is now updateReq which includes a fieldmaskpb.FieldMask.

The tests currently fail due to duplicate proto registration. The author plans to merge this into trunk and address the test failures later.

Happy reviewing!

Code flows like a stream,
Version two, a smoother ride,
Tests will fix the bugs.

This pull request updates the Pub/Sub samples to use the v2 library. The changes are necessary and well-implemented. However, there are a few minor suggestions to improve code readability and maintainability. I have referenced the Google Go Style Guide when making these suggestions.