[8.17](backport #4692) [apm] Clarify how data is populated in the APM Infrastructure tab

[mergify]

Description

Clarifies how data is populated in the Infrastructure tab for a selected service in the Applications UI. This PR also attempts to provide guidance on how to self-diagnose / troubleshoot when the Infrastructure tab does not have any data.

I searched through some GitHub issues and discussion threads in addition to the resources @lucabelluccini linked to in #4359, but I wasn't able to find much information on troubleshooting.

This PR definitely needs more work, but I thought I'd open it to have something for the product/dev teams to respond to. @roshan-elastic can you help me find a technical reviewer? @lucabelluccini does this align with what you were expecting when you opened #4359?

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Closes #4359

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

---

This is an automatic backport of pull request #4692 done by [Mergify](https://mergify.com).

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