[Request]: Document new Timeslice Metric indicator for SLOs

[dedemorton]

Description

Reported by @grabowskit The new timeslice metric indicator added in 8.12 is not documented. We need to add it to the SLO documentation here:

• https://www.elastic.co/guide/en/observability/master/slo-create.html
• https://docs.elastic.co/serverless/observability/create-an-slo

Resources

This feature was implemented in elastic/kibana#168539.

Which documentation set does this change impact?

Stateful and Serverless

Feature differences

Need to confirm this, but I believe the feature is identical in stateful and serverless.

What release is this request related to?

8.12

Collaboration model

The documentation team

Point of contact.

Main contact: TBD

Stakeholders:

[dedemorton]

Reopening because this needs to be copied to serverless, too.