CHANGELOG/CHANGELOG-3.5.md: update changelog

In this commit, we update the changelog for gRPC
metadata printing in release 3.5 which is mainly
a backport from PR etcd-io#18312.

Signed-off-by: Mohamed Awnallah <mohamedmohey2352@gmail.com>

CHANGELOG/CHANGELOG-3.5.md

@@ -334,6 +334,10 @@ See [code changes](https://github.com/etcd-io/etcd/compare/v3.4.0...v3.5.0) and
 
 **Again, before running upgrades from any previous release, please make sure to read change logs below and [v3.5 upgrade guide](https://etcd.io/docs/latest/upgrades/upgrade_3_5/).**
 
+### Improved
+
+- [print gRPC metadata in guarnteed order using the official go fmt pkg](https://github.com/etcd-io/etcd/pull/18312).
+
 ### Breaking Changes
 
 - `go.etcd.io/etcd` Go packages have moved to `go.etcd.io/etcd/{api,pkg,raft,client,etcdctl,server,raft,tests}/v3` to follow the [Go modules](https://github.com/golang/go/wiki/Modules) conventions