Handle logs with complex attributes (not body)

From https://github.com/open-telemetry/semantic-conventions/issues/1651 OTel is moving toward just using OTel LogEntry.attributes and not using the body.
- The GenAI semantic conventions are already following this, since everything goes in attributes https://github.com/open-telemetry/semantic-conventions/blob/v1.37.0/docs/gen-ai/gen-ai-events.md#event-eventgen_aiclientinferenceoperationdetails
- OTel python's LogHandler puts the extra param from `logging.info(..., extra={...})` (structured log message) into attributes ([code](https://github.com/open-telemetry/opentelemetry-python/blob/5c8269c2d33617cca26c63e8627aec89e0d79b17/opentelemetry-sdk/src/opentelemetry/sdk/_logs/_internal/__init__.py#L576-L578), and [test case](https://github.com/open-telemetry/opentelemetry-python/blob/5c8269c2d33617cca26c63e8627aec89e0d79b17/opentelemetry-sdk/tests/logs/test_handler.py#L127-L133)).

This doesn't work well for Cloud Logging because labels only support string values and are expected to be small. We still need some way to export this data though.