Add Retry-After to M_LIMIT_EXCEEDED error responses (#16136)

Implements MSC4041 behind an experimental configuration flag.

tests/api/test_errors.py

@@ -0,0 +1,36 @@
+# 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
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from synapse.api.errors import LimitExceededError
+
+from tests import unittest
+
+
+class ErrorsTestCase(unittest.TestCase):
+    # 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)
+        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)
+        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")