Fix rate limit metrics registering twice and misreporting (#13649)

* Fix rate limit metrics registering twice and misreporting

Fix https://github.com/matrix-org/synapse/issues/13641

* Fix lints

* Add changelog

* Document `metrics_name=None`.

1 files changed, 3 insertions, 1 deletions

diff --git a/synapse/server.py b/synapse/server.py

index 181984a1a4..c2e55bf0b1 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -756,7 +756,9 @@ class HomeServer(metaclass=abc.ABCMeta):
     @cache_in_self
     def get_federation_ratelimiter(self) -> FederationRateLimiter:
         return FederationRateLimiter(
-            self.get_clock(), config=self.config.ratelimiting.rc_federation
+            self.get_clock(),
+            config=self.config.ratelimiting.rc_federation,
+            metrics_name="federation_servlets",
         )
 
     @cache_in_self