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 #18312.

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

CHANGELOG/CHANGELOG-3.5.md

@@ -15,6 +15,9 @@ Previous change logs can be found at [CHANGELOG-3.4](https://github.com/etcd-io/
 - Compile binaries using [go 1.21.12](https://github.com/etcd-io/etcd/pull/18271).
 - [Fully address CVE-2023-45288 and fix govulncheck CI check](https://github.com/etcd-io/etcd/pull/18170)
 
+### Package clientv3
+- [Print gRPC metadata in guarnteed order using the official go fmt pkg](https://github.com/etcd-io/etcd/pull/18312).
+
 ## v3.5.14 (2024-05-29)
 
 ### etcd server