improve instrumentation failure error message and add test

Author: outergod

instrumentation/opentelemetry-instrumentation-fastapi/src/opentelemetry/instrumentation/fastapi/__init__.py

@@ -334,7 +334,9 @@ async def __call__(
                 ):
                     # Oops, something changed about how Starlette creates middleware stacks
                     _logger.error(
-                        "Cannot instrument FastAPI as the expected middleware stack has changed"
+                        "Skipping FastAPI instrumentation due to unexpected middleware stack: expected %s, got %s",
+                        ServerErrorMiddleware.__name__,
+                        type(inner_server_error_middleware),
                     )
                     return inner_server_error_middleware
 

instrumentation/opentelemetry-instrumentation-fastapi/tests/test_fastapi_instrumentation.py

@@ -2064,3 +2064,7 @@ def test_no_instrumentation(self):
             if record.levelno >= logging.ERROR
         ]
         self.assertEqual(len(errors), 1)
+        self.assertEqual(
+            errors[0].getMessage(),
+            "Skipping FastAPI instrumentation due to unexpected middleware stack: expected ServerErrorMiddleware, got <class 'fastapi.routing.APIRouter'>",
+        )