Do not assume hasattr is available in Metric.__del__

Python does not guarantee that builtins are available by the time __del__
is called, so using hasattr is unsafe.

PiperOrigin-RevId: 254255638

tensorflow/python/eager/monitoring.py

@@ -121,10 +121,14 @@ class Metric(object):
     self._metric = self._metric_methods[self._label_length].create(*args)
 
   def __del__(self):
-    if hasattr(self, '_metric'):
+    try:
       deleter = self._metric_methods[self._label_length].delete
-      if deleter is not None:
-        deleter(self._metric)
+      metric = self._metric
+    except AttributeError:
+      return
+
+    if deleter is not None:
+      deleter(metric)
 
   def get_cell(self, *labels):
     """Retrieves the cell."""