fix log formatting

Logs from httpx's `_send_single_request` is not formatted correctly,
resulting logs like 'HTTP Request: %s %s "%s %d %s"'.
This makes debugging network-related issues harder (e.g. a mistyped
3rd party openai api url).
Fixes it by assessing log message via `LogRecord.getMessage()`.
see also: https://docs.python.org/3.10/library/logging.html#logging.LogRecord.getMessage