Metrics declared at toplevel of program interfere with autoreload

To reproduce:

1. Create a file bar.py with contents

from prometheus_client import Gauge

my_gauge = Gauge('my_gauge', 'my description')

def add(x, y):
    return x + y

2. Create a Jupyter notebook with the contents

%load_ext autoreload
%autoreload 2
from bar import add
# ---- cell break ----
add(2, 3)

and run it.
3. Edit bar.py in some way (e.g. add a product(x, y) function).
4. Re-run the add(2, 3) cell in the notebook

Expected behavior

Autoreload works fine; the cell runs without issue.

Actual behavior

After running add(2, 3) a second time, it prints this error to stderr:

[autoreload of bar failed: Traceback (most recent call last):
  File "/dev/shm/uid-21748-gid-32786/39d5b5d2-seed-nspid4026531836-ns-4026531841/lib/python3.10/site-packages/IPython/extensions/autoreload.py", line 245, in check
    superreload(m, reload, self.old_objects)
  File "/dev/shm/uid-21748-gid-32786/39d5b5d2-seed-nspid4026531836-ns-4026531841/lib/python3.10/site-packages/IPython/extensions/autoreload.py", line 394, in superreload
    module = reload(module)
  File "/dev/shm/uid-21748-gid-32786/39d5b5d2-seed-nspid4026531836-ns-4026531841/lib/python3.10/imp.py", line 315, in reload
    return importlib.reload(module)
  File "/dev/shm/uid-21748-gid-32786/39d5b5d2-seed-nspid4026531836-ns-4026531841/lib/python3.10/importlib/__init__.py", line 169, in reload
    _bootstrap._exec(spec, module)
  File "<frozen importlib._bootstrap>", line 619, in _exec
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/j/igm/user/kstaley/pub/Jupyter/play/bar.py", line 3, in <module>
    my_gauge = Gauge('my_gauge', 'my description')
  File "/dev/shm/uid-21748-gid-32786/39d5b5d2-seed-nspid4026531836-ns-4026531841/lib/python3.10/site-packages/prometheus_client/metrics.py", line 365, in __init__
    super().__init__(
  File "/dev/shm/uid-21748-gid-32786/39d5b5d2-seed-nspid4026531836-ns-4026531841/lib/python3.10/site-packages/prometheus_client/metrics.py", line 143, in __init__
    registry.register(self)
ValueError: Duplicated timeseries in CollectorRegistry: {'my_gauge'}
]

Notes

I think instantiating a metric should be idempotent. If you are passing the exact same metric name, docstring, labels, etc., you should get back the same metric object instance. This would imply having a factory function instead of directly exposing the metric classes. Maybe this behavior could be opt-in if enabling it by default is worrisome.

bar.py、IPython autoreload を使用する Jupyter ノートブック、および prometheus_client.Gauge で問題を再現し、その後 duplicate CollectorRegistry の traceback を調べる。トップレベルのメトリクスを再読み込みする際の意図された動作を判断する。done は、bar.py の編集後にノートブックのセルを再実行しても、重複した時系列のエラーなしに完了することを意味する。