ws: Only process a "close" message after forwarding it.

Otherwise, the forwarding will fail because the channel has already
been removed from the knowledge of cockpit-ws.

XXX - this was done in a hurry, and there are still messages like

   cockpit-ws[32211]: channel 1:7 does not exist

in the log.
common: Add cockpit_json_get_array.
bridge: "metrics1" channel.

Initial work by Stef Walter <stefw@redhat.com>.
shell: Use metrics channel for dashboard plots.