Make clear() a no-op for label-less metrics instead of raising AttributeError

Fixes #707. clear() unconditionally acquired self._lock, which is
never created for metrics with no labelnames, causing an
AttributeError. Per discussion in the issue (csmarchbanks,
roidelapluie, sdfordham, tomprince), a label-less metric has no
labelsets to clear, so clear() now treats this as a no-op rather
than raising.

Signed-off-by: Om Pathak <ompathak@gmail.com>

Author: Om Pathak

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:
+            return
         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

@@ -59,6 +59,12 @@ def test_reset(self):
     def test_repr(self):
         self.assertEqual(repr(self.counter), "prometheus_client.metrics.Counter(c)")
 
+    def test_clear_without_labels_is_noop(self):
+        self.counter.inc()
+        self.assertEqual(1, self.registry.get_sample_value('c_total'))
+        self.counter.clear()  # should not raise
+        self.assertEqual(1, self.registry.get_sample_value('c_total'))
+
     def test_negative_increment_raises(self):
         self.assertRaises(ValueError, self.counter.inc, -1)