Currently the metrics plugin uses a really hackish host mount with
propagated mounts to get the metrics socket into a plugin after the
plugin is alreay running.
This approach ends up leaking mounts which requires setting the plugin
manager root to private, which causes some other issues.
With this change, plugin subsystems can register a set of modifiers to
apply to the plugin's runtime spec before the plugin is ever started.
This will help to generalize some of the customization work that needs
to happen for various plugin subsystems (and future ones).
Specifically it lets the metrics plugin subsystem append a mount to the
runtime spec to mount the metrics socket in the plugin's mount namespace
rather than the host's and prevetns any leaking due to this mount.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
426e610e43