fix: raise ValueError when clear() called on metric without labels (#1140)

Metrics without labels never initialize _lock or _metrics (only parent
metrics with labelnames do). Calling clear() on such a metric raised an
unhelpful AttributeError. Guard with the same ValueError pattern that
remove() already uses, matching maintainer guidance that clear() is not
intended for label-less metrics.

Fixes #1140. Also addresses duplicate #949.

Author: kimjune01

prometheus_client/metrics.py

@@ -254,6 +254,8 @@ def remove_by_labels(self, labels: dict[str, str]) -> None:
 
     def clear(self) -> None:
         """Remove all labelsets from the metric"""
+        if not self._labelnames:
+            raise ValueError('No label names were set when constructing %s' % self)
         if 'prometheus_multiproc_dir' in os.environ or 'PROMETHEUS_MULTIPROC_DIR' in os.environ:
             warnings.warn(
                 "Clearing labels has not been implemented in multi-process mode yet",

tests/test_core.py

@@ -620,6 +620,12 @@ def test_clear(self):
         self.assertEqual(None, self.registry.get_sample_value('c_total', {'l': 'x'}))
         self.assertEqual(None, self.registry.get_sample_value('c_total', {'l': 'y'}))
 
+    def test_clear_no_labels_raises(self):
+        """clear() on a metric without labels should raise ValueError, not AttributeError."""
+        no_labels = Counter('c_no_labels', 'help', registry=self.registry)
+        no_labels.inc()
+        self.assertRaises(ValueError, no_labels.clear)
+
     def test_incorrect_label_count_raises(self):
         self.assertRaises(ValueError, self.counter.labels)
         self.assertRaises(ValueError, self.counter.labels, 'a', 'b')