- Add alert rule for sentry.gitlab.com
 - Use runbook troubleshooting/sentry-is-down.md
 - Fixes https://gitlab.com/gitlab-com/infrastructure/issues/1237

The update of taking point on an issue should notify all active members in the channel with an alert.

Our new recovery.conf generation procedure doesn't create trigger files,
so we should only document pg_ctl usage.

Additional recording rules for improving dashboards speed.

Include the example rules from the node_exporter[0].

[0]: https://github.com/prometheus/node_exporter/blob/master/example.rules

1. Moved from hours to seconds
2. Fixed some typos
3. Humanize output