update metrics on successful install

[jshufro]

If the update tracker is installed, smartnode-install should quietly fork off

/usr/share/apt-metrics.sh | sponge /var/lib/node_exporter/textfile_collector/apt.prom

and suppress any errors it might emit.

this will hopefully clear the metric (though I'm on mobile and can't check whether we look at service version or binary version- might need a service start first, in which case, this whole ticket is moot)