This restores the config that is usable for prometheus pre v2.0.0
The new config only works for Prometheus v2+

Starting with synctl lead to "no config file found"
Stopping also leads to some (code=exited, status=1/FAILURE), but at least now we can stop the service.

Non-existing files, when running the suggested from https://github.com/matrix-org/synapse#configuring-synapse
/etc/synapse/log_config.yaml so the --log-config leads to an error
/etc/sysconfig/synapse The environment-file or even the /etc/sysconfig does not exist in arch linux

Also instead of calling python2 we use synctl, as this seems to be the proper way to start it, and it gives us a more useful error in the systemctl status. And we now allow stop (and therefore restart).

... from https://github.com/matrix-org/synapse-prometheus-config.

The documentation on get_json has been wrong ever since the very first commit
to synapse...