ratelimit: Allocate headers only if ratelimit service sends non-empty headers. (#8640)

Signed-off-by: Nandu Vinodan <nandu.vinodan@freshworks.com>

Mirrored from https://github.com/envoyproxy/envoy @ a55621cb23f69aac44a6a1fc5ba1ffd1475293e5

envoy/service/ratelimit/v2/rls.proto

@@ -101,6 +101,7 @@ message RateLimitResponse {
   // descriptors failed and/or what the currently configured limits are for all of them.
   repeated DescriptorStatus statuses = 2;
 
+  // A list of headers to add to the response
   // [#next-major-version: rename to response_headers_to_add]
   repeated api.v2.core.HeaderValue headers = 3;
 

envoy/service/ratelimit/v3alpha/rls.proto

@@ -101,6 +101,7 @@ message RateLimitResponse {
   // descriptors failed and/or what the currently configured limits are for all of them.
   repeated DescriptorStatus statuses = 2;
 
+  // A list of headers to add to the response
   // [#next-major-version: rename to response_headers_to_add]
   repeated api.v3alpha.core.HeaderValue headers = 3;