rate limit: support match input functions (#21184)

Signed-off-by: Kuat Yessenov <kuat@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ afabff514aba3c312b07c60779b6a4c7a42eb54a

envoy/config/route/v3/route_components.proto

@@ -1976,6 +1976,12 @@ message RateLimit {
       MetaData metadata = 8;
 
       // Rate limit descriptor extension. See the rate limit descriptor extensions documentation.
+      //
+      // :ref:`HTTP matching input functions <arch_overview_matching_api>` are
+      // permitted as descriptor extensions. The input functions are only
+      // looked up if there is no rate limit descriptor extension matching
+      // the type URL.
+      //
       // [#extension-category: envoy.rate_limit_descriptors]
       core.v3.TypedExtensionConfig extension = 9;