List unofficial Common Lisp client

https://github.com/deadtrickster/prometheus.cl

content/docs/instrumenting/clientlibs.md

@@ -21,6 +21,7 @@ HTTP endpoint on your application’s instance:
 Unofficial third-party client libraries:
 
 * [Bash](https://github.com/aecolley/client_bash)
+* [Common Lisp](https://github.com/deadtrickster/prometheus.cl)
 * [Haskell](https://github.com/fimad/prometheus-haskell)
 * [Lua](https://github.com/knyar/nginx-lua-prometheus) for Nginx
 * [.NET / C#](https://github.com/andrasm/prometheus-net)