Find a specific cgroup file, containing metrics to extract.

def _get_cgroup_from_proc(self, cgroup, pid, filename):
        """Find a specific cgroup file, containing metrics to extract."""
        params = {
            "file": filename,
        }
        return DockerUtil.find_cgroup_from_proc(self._mountpoints, pid, cgroup, self.docker_util._docker_root) % (params)