fix test by adding raw property in mock response (Azure#21577)

Author: xiangyan99

sdk/monitor/azure-monitor-opentelemetry-exporter/tests/test_base_exporter.py

@@ -316,3 +316,8 @@ def __init__(self, status_code, text, headers={}, reason="test", content="{}"):
         self.headers = headers
         self.reason = reason
         self.content = content
+        self.raw = MockRaw()
+
+class MockRaw:
+    def __init__(self):
+        self.enforce_content_length = False