Add GDSMetrics package

As per instructions https://github.com/alphagov/gds_metrics_python

The celery workers don't have an HTTP endpoint so no point in trying to
get prometheus to scrape them.

gunicorn_config.py

@@ -3,6 +3,8 @@ import sys
 import traceback
 import gunicorn
 
+from gds_metrics.gunicorn import child_exit  # noqa
+
 workers = 4
 worker_class = "eventlet"
 worker_connections = 256