(Prometheus) Pushgateway

This is a hack to get pushgateway into a container using pack so it can be deployed on Toolforge.

It is slightly less terrible than the previous solution of dumping the pre-compiled binary onto NFS then executing it in a container.

Once T401075 / T363027 is resolved, this can be replaced.

Logic

• setup.py handles setting up the container image (downloading the binary)
• entrypoint.py handles setting up the runtime (config)

Testing locally

$ pack build --builder heroku/builder:24 external-pushgateway