Change dependency to prometheus_client_php

[danvirsen]

https://github.com/Jimdo/prometheus_client_php is archived and no longer maintained.
They didn't add the abandoned value in composer.json, so there's no link to it from there, but my suggestion is to change the dependency to the one they link to in the repository readme: https://github.com/endclothing/prometheus_client_php.