mautrix-telegram: add `prometheus-client` for metrics

With this change I can do

    metrics:
        enabled: true
        listen_port: 8080

and retrieve metrics from `localhost:8080` for the telegram bridge.

pkgs/servers/mautrix-telegram/default.nix

@@ -54,6 +54,7 @@ in python.pkgs.buildPythonPackage rec {
     pillow
     lxml
     setuptools
+    prometheus-client
   ] ++ lib.optionals withE2BE [
     asyncpg
     python-olm