Since backups may now taken on arbitrary hosts, we need a blackbox
monitor that _some_ backup was produced.
Add a Prometheus exporter which calls `wal-g backup-list` and reports
statistics about the backups.
This could be extended to include `wal-g wal-verify`, but that
requires a connection to the PostgreSQL server.
In the event that extracting doesn't produce the binary we expected it
to, all this will do is create an _empty_ file where we expect the
binary to be. This will likely muddle debugging.
Since the only reason the resourfce was made in the first place was to
make dependencies clear, switch to depending on the External_Dep
itself, when such a dependency is needed.
This is similar cleanup to 3ab9b31d2f, but only affects zulip_ops
services; it serves to ensure that any of these services which are no
longer enabled are automatically removed from supervisor.
Note that this will cause a supervisor restart on all affected hosts,
which will restart all supervisor services.
Alex Vandiver