opentelemetry-test-utils: don't crash in TestBase.get_sorted_metrics …

…without metrics (#4194)

tests/opentelemetry-test-utils/src/opentelemetry/test/test_base.py

@@ -144,8 +144,9 @@ def disable_logging(highest_level=logging.CRITICAL):
             logging.disable(logging.NOTSET)
 
     def get_sorted_metrics(self):
+        metrics_data = self.memory_metrics_reader.get_metrics_data()
         resource_metrics = (
-            self.memory_metrics_reader.get_metrics_data().resource_metrics
+            metrics_data.resource_metrics if metrics_data else []
         )
 
         all_metrics = []

tests/opentelemetry-test-utils/tests/test_base.py

@@ -0,0 +1,21 @@
+# Copyright The OpenTelemetry Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from opentelemetry.test.test_base import TestBase
+
+
+class TestBaseTestCase(TestBase):
+    def test_get_sorted_metrics_works_without_metrics(self):
+        metrics = self.get_sorted_metrics()
+        self.assertEqual(metrics, [])