Make default meter a DefaultMeter

opentelemetry-api/src/opentelemetry/metrics/__init__.py

@@ -347,7 +347,12 @@ def meter() -> Meter:
 
     if _METER is None:
         # pylint:disable=protected-access
-        _METER = loader._load_impl(Meter, _METER_FACTORY)
+        try:
+            _METER = loader._load_impl(Meter, _METER_FACTORY)
+        except TypeError:
+            # if we raised an exception trying to instantiate an
+            # abstract class, default to no-op tracer impl
+            _METER = DefaultMeter()
         del _METER_FACTORY
 
     return _METER

opentelemetry-api/tests/metrics/test_metrics.py

@@ -13,6 +13,8 @@
 # limitations under the License.
 
 import unittest
+from contextlib import contextmanager
+from unittest import mock
 
 from opentelemetry import metrics
 
@@ -90,3 +92,33 @@ def test_gauge_handle(self):
     def test_measure_handle(self):
         handle = metrics.MeasureHandle()
         handle.record(1)
+
+
+@contextmanager
+def patch_metrics_globals(meter=None, meter_factory=None):
+    """Mock metrics._METER and metrics._METER_FACTORY.
+
+    This prevents previous changes to these values from affecting the code in
+    this scope, and prevents changes in this scope from leaking out and
+    affecting other tests.
+    """
+    with mock.patch("opentelemetry.metrics._METER", meter):
+        with mock.patch("opentelemetry.metrics._METER_FACTORY", meter_factory):
+            yield
+
+
+class TestGlobals(unittest.TestCase):
+    def test_meter_default_factory(self):
+        """Check that the default meter is a DefaultMeter."""
+        with patch_metrics_globals():
+            meter = metrics.meter()
+            self.assertIsInstance(meter, metrics.DefaultMeter)
+            # Check that we don't create a new instance on each call
+            self.assertIs(meter, metrics.meter())
+
+    def test_meter_custom_factory(self):
+        """Check that we use the provided factory for custom global meters."""
+        mock_meter = mock.Mock(metrics.Meter)
+        with patch_metrics_globals(meter_factory=lambda _: mock_meter):
+            meter = metrics.meter()
+            self.assertIs(meter, mock_meter)