Allows for a plugin type that can be used to scrape metrics.
This is useful because metrics are not neccessarily at a standard
location... `--metrics-addr` must be set, and must currently be a TCP
socket.
Even if metrics are done via a unix socket, there's no guarentee where
the socket may be located on the system, making bind-mounting such a
socket into a container difficult (and racey, failure-prone on daemon
restart).
Metrics plugins side-step this issue by always listening on a unix
socket and then bind-mounting that into a known path in the plugin
container.
Note there has been similar work in the past (and ultimately punted at
the time) for consistent access to the Docker API from within a
container.
Why not add metrics to the Docker API and just provide a plugin with
access to the Docker API? Certainly this can be useful, but gives a lot
of control/access to a plugin that may only need the metrics. We can
look at supporting API plugins separately for this reason.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
