[Improve][SDK] Update message metrics when no available worker in the DataProxy Go SDK

### Description

In the Dataproxy Go SDK, messages rejected with ErrNoAvailableWorker in Send/SendAsync are not reflected in the message metrics, so these failures are invisible in monitoring.

### InLong Component

InLong SDK

### Are you willing to submit PR?

- [x] Yes, I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Start in the DataProxy Go SDK at the Send and SendAsync entry points, and trace how ErrNoAvailableWorker is handled. Compare those failure paths with existing message-metric updates; done means rejected messages are represented in the metrics for both synchronous and asynchronous sends.

Written by the indexing model from the issue text.