Allow service to provide additional status information

[Marcus]

Currently, the initctl status <job> / service <service> status command displays only the pid and the current init state as the status of the service. With non-trivial services, this is hardly sufficient to determine the actual state of the service. For a developer it would be nice if there was an API or some other way (file descriptor/PIPE/whatever) that could be utilized to send additional status information about a running service back to upstart to provide admins with a more complete picture on how the service is actually doing.

Examples of such information could be:
 - Avg. network traffic of service
 - Currently enabled/disabled plugins
 - Number of users currently using the service
 - Port numbers of randomly opened UDP listening ports

Something similar is currently supported by systemd[1]. It would of course be awesome if upstart and systemd could find a compatible API to achieve this.

[1] http://www.freedesktop.org/software/systemd/man/sd_notify.html