Document how to retain data ILM policies

[andrewvc]

Now that we have separate ILM policies for each type of monitor (elastic/uptime#462), monitors running through Elastic Agent that make use of datastreams (which includes the Synthetics Service, and self-managed nodes elastic/uptime#441), we should document how users can customise these to suit their requirements (e.g longer or shorter retentions based on the kind of data).

Considerations:

• Where should the entry point be for this documentation?
  • Monitor Management is a temporary UI, so perhaps we should show how to configure these for PUSH or Synthetics UI based monitors, or those running from self-managed nodes
  • The existing Synthetics Integration will be deprecated and removed, in favour of the self-managed nodes, so probably not worth documenting that workflow
  • Perhaps the existing ILM / policy docs are the best place to highlight this (with a link from the Synthetics docs)
• Docs should include some indication of how the different types of data for different monitor types have a different storage profile (e.g. we know that the network data can take up the most space, and then screenshots)

[paulb-elastic]

Lower priority than elastic/integrations#2196

[paulb-elastic]

This should wait for elastic/uptime#453 to be done, in case that identifies changes to the way we configure data streams to ILM policies.

[paulb-elastic]

This should be based on the implementation in elastic/uptime#462

[paulb-elastic]

Rather than the original Synthtics Integration, I think we should re-purpose this to do it based on the elastic/uptime#462 implementation (for the Synthetics Service), and then also the new integration we'll get with elastic/uptime#441

[paulb-elastic]

Updated this issue description as discussed in the docs sync yesterday

[paulb-elastic]

Discussed with @dominiqueclarke about the placement and content, and she suggested

I think the documentation should either restate the information from the fleet document with a focus on the synthetics data streams, or link to the fleet documentation.