Client Telemetry for Pub/Sub Publish Operations

The pubsub.v1beta2 API now includes two new schemas. PubsubClientTelemetry is introduced to capture client-side telemetry, intended to be passed via the x-goog-pubsub-client-telemetry header. This schema includes a publishOperation field, which references the new PublishOperation schema. The PublishOperation schema itself now exposes hedgedAttemptCount (an integer indicating hedging attempts) and publishStartTime (a timestamp for the client-side initiation of the publish call), offering deeper insights into client-side publish behavior and performance.