Add unreleased changelog entry

Author: xrmx

CHANGELOG.md

@@ -5,6 +5,8 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
 ## Version 1.32.0/0.53b0 (2025-04-10)
 
 - Fix user agent in OTLP HTTP metrics exporter