Update span (partial)

## Authentication - API key: `Authorization: Bearer ` Request body: ```json { "metadata": { "user_id": "user456", "rating": 5 } } ``` Request example: ```bash curl -X PATCH "https://api.respan.ai/api/datasets/{dataset_id}/logs/{log_id}/" \ -H "Authorization: Bearer " \ -H "Content-Type: application/json" \ -d '{ "metadata": { "user_id": "user456", "rating": 5 } }' ``` Response (200 OK): ```json { "id": "log_12345", "dataset_id": "dataset_67890", "messages": [ { "role": "user", "content": "What is the capital of France?" }, { "role": "assistant", "content": "The capital of France is Paris." } ], "model": "gpt-4", "parameters": { "temperature": 0.7, "max_tokens": 150 }, "metadata": { "user_id": "user456", "session_id": "session456", "rating": 5 }, "created_at": "2024-01-15T10:30:00Z", "updated_at": "2024-01-15T11:45:00Z" } ``` Errors: - 401 Unauthorized — Missing/invalid authentication - 404 Not Found — Dataset or log not found - 400 Bad Request — Invalid request body