Add _metadata.get_project_id.
diff --git a/tests/compute_engine/test__metadata.py b/tests/compute_engine/test__metadata.py
index 4cc3e55..6a45c42 100644
--- a/tests/compute_engine/test__metadata.py
+++ b/tests/compute_engine/test__metadata.py
@@ -126,6 +126,20 @@
         headers=_metadata._METADATA_HEADERS)
 
 
+def test_get_project_id(mock_request):
+    project = 'example-project'
+    request_mock = mock_request(
+        project, headers={'content-type': 'text/plain'})
+
+    project_id = _metadata.get_project_id(request_mock)
+
+    request_mock.assert_called_once_with(
+        method='GET',
+        url=_metadata._METADATA_ROOT + 'project/project-id',
+        headers=_metadata._METADATA_HEADERS)
+    assert project_id == project
+
+
 @mock.patch('google.auth._helpers.utcnow', return_value=datetime.datetime.min)
 def test_get_service_account_token(utcnow, mock_request):
     ttl = 500
diff --git a/tests/test__default.py b/tests/test__default.py
index 747bfa4..e244b3d 100644
--- a/tests/test__default.py
+++ b/tests/test__default.py
@@ -214,7 +214,8 @@
 @mock.patch(
     'google.auth.compute_engine._metadata.ping', return_value=True)
 @mock.patch(
-    'google.auth.compute_engine._metadata.get', return_value='example-project')
+    'google.auth.compute_engine._metadata.get_project_id',
+    return_value='example-project')
 def test__get_gce_credentials(get_mock, ping_mock):
     credentials, project_id = _default._get_gce_credentials()
 
@@ -233,7 +234,7 @@
 @mock.patch(
     'google.auth.compute_engine._metadata.ping', return_value=True)
 @mock.patch(
-    'google.auth.compute_engine._metadata.get',
+    'google.auth.compute_engine._metadata.get_project_id',
     side_effect=exceptions.TransportError())
 def test__get_gce_credentials_no_project_id(get_mock, ping_mock):
     credentials, project_id = _default._get_gce_credentials()