disable prometheus writing to files from celery apps

## The existing situation

To support multiple processes and eventlets recording metrics in
parallel, prometheus uses files to store metrics. When you write a
metric from a multiprocess app, it writes to a file.

Prometheus identifies whether your app is multiprocess by looking for
the existence of a `prometheus_multiproc_dir` environment var (in either
case). Prometheus reads this variable at a module level (ie: at import
time). Assuming it will always used within a web server, the gds_metrics
library auto-sets this to `/tmp` on import, to ensure that prometheus
will always be set up correctly.

We also have a variety of metrics set up when we create the app. These
are generally sensible metrics such as counting the number of database
connections in use by measuring sqlalchemy connection events.

## The problem

We have seen problems with our notify-delivery-worker-reporting app run
out of space. The CELERYD_MAX_TASKS_PER_CHILD flag is set on that app
which restarts each worker process every time a task runs (to avoid
memory issues), however we've recently massively decreased the size and
increased the number of tasks to parallelise nightly tasks. Each time a
worker process restarts it will write a new file to disk. This meant
that we quickly ran out of disc space, and then the entire app instance
was killed.

The big rub is that we don't log prometheus metrics from our worker
apps! They don't expose an endpoint so there's no way to scrape them so
we aren't getting any value from prometheus anyway! But because they use
the same codebase they import gds_metrics and get that anyway.

## The solution

gds_metrics sets the multiproc env var, however, by importing prometheus
FIRST we ensure that the env var is unset at that point, and thus
prometheus will harmlessly store the metrics in memory.

To ensure that when we run the notify-api that still has the env var set
so the stats are shared across all the gunicorn processes, we put this
import as the first thing in run_celery.py

run_celery.py

@@ -2,6 +2,11 @@
 
 from flask import Flask
 
+# import prometheus before any other code. If gds_metrics is imported first it will write a prometheus file to disk
+# that will never be read from (since we don't have prometheus celery stats). If prometheus is imported first,
+# prometheus will simply store the metrics in memory
+import prometheus_client  # noqa
+
 # notify_celery is referenced from manifest_delivery_base.yml, and cannot be removed
 from app import notify_celery, create_app  # noqa