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)

Riproduci l’esempio minimo dell’issue, quindi esamina il metodo clear in metrics.py alla riga 213 e il percorso di inizializzazione di un Gauge senza label. Conferma che chiamare clear() su una metrica senza label dichiarate non sollevi più l’AttributeError riportato e che il caso con label continui a funzionare.