Pub/Sub Adds Client Telemetry for Publish Operations

Two new schemas, PubsubClientTelemetry and PublishOperation, have been added to the Pub/Sub API. PubsubClientTelemetry is designed to carry client-side telemetry data, which will be sent via the x-goog-pubsub-client-telemetry header. It includes a publishOperation field, referencing the new PublishOperation schema. The PublishOperation schema provides details like hedgedAttemptCount (an integer indicating hedging attempts) and publishStartTime (a google-datetime string representing the client-side initiation time of the publish call). These new fields offer developers more visibility into the behavior and performance of their Pub/Sub publish calls, particularly for debugging client library interactions and understanding end-to-end latency with hedging.