[Synthetics] Document retest feature

[paulb-elastic]

Description

From 8.11 onwards, the default behaviour for Synthetics will be to automatically retest if the monitor (lightweight and/or browser) goes from an UP condition to DOWN. If this retest is also DOWN, then the error will be created, and if configured, the alert sent.

This is to prevent noisy errors/alerts for transient problems. See elastic/synthetics#792 for more details.

• This feature required Kibana to be running 8.11.0+
• Heartbeat/Synthetics will be upgraded to support retest, on the service, before 8.11 is released
• Anyone running < 8.11.0 Kibana will still have the existing behaviour
• For private locations, this will also become the default behaviour if running Elastic Agent 8.11.0+
• Users can disable the default behaviour in the UI, or via project monitor configuration
• The UI also includes some additional info and filters to identify which tests are retests
• With this behaviour, errors are only created after the first retest (and if we get DOWN, DOWN), which would trigger the alert

It's also worth documenting somewhere, that monitors can be a maximum of 15 minutes per run, at which point they will be terminated (for any browser test, and a retest will allow 2x 15 minute runs, when the second is a retest).

Resources

The main feature is described in elastic/synthetics#792, with the Kibana changes done in elastic/kibana#162399.

A new version of the Synthetics Agent is needed (planned in 1.5.0) that will include elastic/synthetics#845, which allows this default behaviour to be toggled off for lightweight and/or browser based project monitors.

Collaboration

The documentation team will investigate the issue and create the initial content.

Point of contact.

Main contact: @shahzad31

Stakeholders:

• @drewpost
• @paulb-elastic