chore: release v0.15.21

Author: unleash-bot[bot]

CHANGELOG.md

@@ -7,6 +7,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [0.15.21](https://github.com/Unleash/unleash-types-rs/compare/v0.15.20...v0.15.21) - 2025-09-29
+
+### 🚀 Features
+- histogram metric support ([#123](https://github.com/unleash/unleash-types-rs/issues/123)) (by @kwasniew) - #123
+
+### Contributors
+
+* @kwasniew
+
 ## [0.15.20](https://github.com/Unleash/unleash-types-rs/compare/v0.15.19...v0.15.20) - 2025-07-31
 
 ### 🐛 Bug Fixes