SCA charm hooks fire stale file during leadership elections

The file age alert for /srv/dashboard.snapcraft.io/production/var/run/store_kpi.state fired on scasnap's sca-app/3 today, and the file was a week old. It turned out sca-app/0 was the leader, so it at first appeared that somehow this check had been switched on in error.

The check soon resolved, because sca-app/3 had just won a leadership election and was taking over that job.

It seems that the check update happens a little too early in the leadership process, and should probably only flip on nearer the end or middle, once the new leader has had a chance to take over properly.