Describe which rate limiter was hit in logs (#16135)

tests/api/test_errors.py

@@ -1,6 +1,5 @@
 # Copyright 2023 The Matrix.org Foundation C.I.C.
 #
-#
 # 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
@@ -13,24 +12,32 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import json
+
 from synapse.api.errors import LimitExceededError
 
 from tests import unittest
 
 
-class ErrorsTestCase(unittest.TestCase):
+class LimitExceededErrorTestCase(unittest.TestCase):
+    def test_key_appears_in_context_but_not_error_dict(self) -> None:
+        err = LimitExceededError("needle")
+        serialised = json.dumps(err.error_dict(None))
+        self.assertIn("needle", err.debug_context)
+        self.assertNotIn("needle", serialised)
+
     # Create a sub-class to avoid mutating the class-level property.
     class LimitExceededErrorHeaders(LimitExceededError):
         include_retry_after_header = True
 
     def test_limit_exceeded_header(self) -> None:
-        err = ErrorsTestCase.LimitExceededErrorHeaders(retry_after_ms=100)
+        err = self.LimitExceededErrorHeaders(limiter_name="test", retry_after_ms=100)
         self.assertEqual(err.error_dict(None).get("retry_after_ms"), 100)
         assert err.headers is not None
         self.assertEqual(err.headers.get("Retry-After"), "1")
 
     def test_limit_exceeded_rounding(self) -> None:
-        err = ErrorsTestCase.LimitExceededErrorHeaders(retry_after_ms=3001)
+        err = self.LimitExceededErrorHeaders(limiter_name="test", retry_after_ms=3001)
         self.assertEqual(err.error_dict(None).get("retry_after_ms"), 3001)
         assert err.headers is not None
         self.assertEqual(err.headers.get("Retry-After"), "4")