feat(pubsub): add OpenTelemetry events and logging for hedged publish

[feywind]

Overview

This PR is Layer 4 of 7 in Stack #9467, which implements Publish Hedging for @google-cloud/pubsub to mitigate high-percentile (p99/p99.9) publish tail latencies by sending backup (hedged) publish RPCs when an in-flight batch takes longer than a configurable hedgeDelay threshold.

Stack Structure

1. feat(gax): support cancelling API calls via CallOptions.signal #9462 - feat(gax): support AbortSignal cancellation in CallOptions and createApiCall
2. feat(pubsub): add HedgingOptions validation and token bucket #9463 - feat(pubsub): add HedgingOptions validation and HedgingTokenBucket
3. feat(pubsub): add CancellationSharer and HedgingScheduler #9464 - feat(pubsub): add CancellationSharer and HedgingScheduler
4. feat(pubsub): add OpenTelemetry events and logging for hedged publish #9465 - feat(pubsub): add OpenTelemetry events and logging for hedged publish (this PR)
5. feat(pubsub): integrate publish hedging into MessageQueue #9466 - feat(pubsub): integrate publish hedging into MessageQueue
6. test(pubsub): add unit tests for publish hedging queue integration #9635 - test(pubsub): add unit tests for publish hedging queue integration
7. test(pubsub): add publish hedging system tests #9636 - test(pubsub): add publish hedging system tests

---

Summary of Changes in This PR

This layer adds OpenTelemetry span events and debug sub-logging for hedged publish attempts, matching the event names used in the Java client implementation:

• OpenTelemetry Span Events (handwritten/pubsub/src/telemetry-tracing.ts):
  • PubsubEvents.publishStartHedged(message) - records 'publish start (hedged)' on the message's parentSpan when a hedged attempt is dispatched.
  • PubsubEvents.publishEndHedged(message) - records 'publish end (hedged)' on the message's parentSpan when a hedged attempt wins the race.
• Debug Sublogger (handwritten/pubsub/src/publisher/message-queues.ts):
  • Added logs.publishHedged = baseLogs.pubsub.sublog('publish-hedged') for logging hedged attempt dispatches and token-bucket rate limiting.
• Unit Tests (handwritten/pubsub/test/telemetry-tracing.ts):
  • Verified that publishStartHedged and publishEndHedged record the expected span events on producer spans.

[gemini-code-assist]

Code Review

This pull request introduces telemetry and logging support for hedged publishing in the Pub/Sub client. Specifically, it adds a new sublog 'publishHedged' to the message queue logs, implements 'publishStartHedged' and 'publishEndHedged' event tracking methods in 'PubsubEvents', and includes corresponding unit tests to verify that these new hedged events are correctly recorded on the publisher span. There are no review comments to address.