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)

Reproduce the minimal example from the issue, then inspect the clear method at metrics.py line 213 and the initialization path for an unlabeled Gauge. Confirm that calling clear() on a metric without declared labels no longer raises the reported AttributeError and that the labeled case remains working.