[apm/known_issues] Add known issues for ILM to DSL switch

[lahsivjar]

APM-Server switched from Index Lifecycle Management(ILM) to Datastream Lifecycle (DSL) in v8.15.0. This switch was done when we moved from APM integration (which used ILM) to APM-data plugin (which uses DSL) in ES for managing APM datastreams. As a result of the switch, any old datastreams created before the switch would be Unmanaged because the datastream will never be updated with the DSL lifecycle -- this has to be done manually by using the PUT API.

Related issue: elastic/apm-server#13898

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[mergify]

This pull request does not have a backport label. Could you fix it @lahsivjar? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[lahsivjar]

[For reviewers] This doc update should be valid for both 8.15.1 and 8.16.0, however, the fix for 8.16.0 will probably diverge and is not in yet. I am not sure if we should only merge this for 8.15 now or for both (given that we MUST fix it for 8.16.0. CC: @simitt @endorama @inge4pres (8.15 release manager)

[simitt]

I would say merge for 8.15.1 now, and for 8.16.0 later when it is actually fixed

[colleenmcginnis]

I left some suggestions below. Let me know if my suggestions affect accuracy.

Also, if there are any specific error messages or behaviors that would signal to the user that they are experiencing this issue, please add them here. That can help get users to this info when they search Google etc. for a specific error message or description of a particular behavior they are seeing.

[simitt]

left some small comments

[colleenmcginnis]

I just suggested some minor copy edits below. 🙂

[inge4pres]

I would say merge for 8.15.1 now, and for 8.16.0 later when it is actually fixed

IIANM we cannot do that, but we can merge this as-is for 8.16, backport to 8.15 and then once the fix is completed for 8.16 update again the main branch to amend the docs for 8.16

[simitt]

I would say merge for 8.15.1 now, and for 8.16.0 later when it is actually fixed

IIANM we cannot do that, but we can merge this as-is for 8.16, backport to 8.15 and then once the fix is completed for 8.16 update again the main branch to amend the docs for 8.16

SGTM!
Vishal is already working on also getting this fixed for 8.16, and based on latest conversations we expect 8.16 to work the same as 8.15.1.

[simitt]

@colleenmcginnis are we ok with merging this in already and also backport before the 8.15.1 release is out?

[colleenmcginnis]

Yep! I can merge now and make sure the backport goes through successfully.