Metrics without labels missing breaks some codepath

When initialized without labels, it seems that the resulting metrics objects will lack some instance variables.

Minimal example:

from prometheus_client import Gauge, generate_latest

cc = Gauge("dummy_count", "POC")

if __name__ == '__main__':
    cc.set(42)
    print(generate_latest().decode())  # OK

    cc.clear()    # raises

"""
Traceback (most recent call last):
  File "prom_example.py", line 10, in <module>
    cc.clear()    # raises
    ^^^^^^^^^^
  File …/site-packages/prometheus_client/metrics.py", line 213, in clear
    with self._lock:
         ^^^^^^^^^^
AttributeError: 'Gauge' object has no attribute '_lock'
"""

The same code with declared labels will properly clear the metrics data

I came up upon this when adding a new metric and forgetting to add the labels.

(I'm using .clear() to reset the metrics during tests)

Reproduisez l’exemple minimal de l’issue, puis examinez la méthode clear dans metrics.py à la ligne 213 ainsi que le chemin d’initialisation d’un Gauge sans labels. Confirmez que l’appel de clear() sur une métrique sans labels déclarés ne lève plus l’AttributeError signalée et que le cas avec labels continue de fonctionner.