(BSR)[API] fix: Restore Gunicorn "pre request" logs

[dbaty]

Registering our own handler replaces the default Gunicorn handler,
which logs the start of the request, like this:

[634677] [DEBUG] GET /health/api

... which is useful to analyze SIGKILL sent to Gunicorn workers (see
PC-27885 for an helper script).

---

Testé en local en lançant Gunicorn comme suit :

$ ENABLE_FLASK_PROMETHEUS_EXPORTER=1 PROMETHEUS_MULTIPROC_DIR=/tmp/pass-culture-prometheus GUNICORN_PORT=4999 GUNICORN_MAX_REQUESTS=10 GUNICORN_MAX_REQUESTS_JITTER=10 GUNICORN_WORKERS=2 GUNICORN_THREADS=2 GUNICORN_TIMEOUT=10 GUNICORN_LOG_LEVEL=debug ./entrypoint.sh
[...]
[2024-06-24 09:52:28 +0200] [634677] [DEBUG] GET /health/api