Duplicated timeseries in CollectorRegistry with Multiprocess Gunicorn

I know this is a subject that comes up somewhat frequently but for the love of me I can't figure out what I'm doing wrong.

1. I have a service in Amazon ECS thats running a single task with multiple workers (actually the problem happens in my other service that just has one worker also).

2. I've created the directory and set the PROMETHEUS_MULTIPROC_DIR in the Dockerfile:

RUN mkdir -p /tmp/prom-metrics
ENV PROMETHEUS_MULTIPROC_DIR /tmp/prom-metrics

3. I'm using the sample code in the README to create the registry in the /metrics request and return it:

registry = CollectorRegistry()
if getenv('PROMETHEUS_MULTIPROC_DIR'):
  multiprocess.MultiProcessCollector(registry)
data = generate_latest(registry)
status = '200 OK'
response_headers = [
    ('Content-type', CONTENT_TYPE_LATEST),
    ('Content-Length', str(len(data))),
]
return Response(data, status, response_headers)

4. I've created the gunicorn.conf.py file with the sample from the README and passed it into my gunicorn startup script via -c:

from prometheus_client import multiprocess

def child_exit(server, worker):
    multiprocess.mark_process_dead(worker.pid)

In my two services, gunicorn starts them as follows:

# app 1 with workers
gunicorn -c /app/utils/gunicorn.conf.py -b :5000 -t 3600 --keep-alive 60 --threads 8 --workers 3 app:app

# app 2 without workers
gunicorn -c /app/utils/gunicorn.conf.py -b :5000 -t 3600 --keep-alive 60 --threads 8 app:app

The service boots successfully and accepts some metrics which are definitely collected in multiprocess mode, seeing as the HELP line simply displays Multiprocess metric.

This works for a few calls but eventually I get the dreaded Duplicated timeseries in CollectorRegistry error and no additional metrics are populated.

What might I be doing wrong?

README の multiprocess metrics の例と、参照されている gunicorn.conf.py 設定から始め、次に示されている Dockerfile の設定と Gunicorn コマンドを使って失敗を再現します。/metrics が引き続きメトリクスを返すように、重複した timeseries エラーの原因を特定し、設定を文書化または修正できれば完了です。