[Bug]: The request-response loogging doesn't include the principal

### File Name

gemini/logging/intro_request_response_logging.ipynb

### What happened?

Logs doesn't contain the principal that requested the endpoint hence there is no way to audit this. The [logging table schema](https://docs.cloud.google.com/gemini-enterprise-agent-platform/models/capabilities/request-response-logging#schema) should not only the project id, but the principal that sent the request.

### Relevant log output

```shell

```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Open gemini/logging/intro_request_response_logging.ipynb and inspect how request-response logging is configured and demonstrated. Compare the notebook's logging table fields with the linked schema, then verify that the resulting logs include the principal alongside the project ID.

Written by the indexing model from the issue text.