Add step for Redis failover script to restart metrics exporter

Just a small improvement to this script that came up while I was doing gitlab-com/gl-infra/production#14511 (closed)

In some instances, metrics stopped getting sent from the reconfigured Redis nodes because process-exporter for whatever reason didn't recognise that Redis had restarted. This script makes restarting process-exporter part of the process so it doesn't need to be done manually.