[8.x](backport #4319) [synthetics] Add fields synthetics configuration option

[mergify]

Description

Add fields configuration option for synthetic monitoring.

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 #4274

Checklist

• Browser monitor config
• Lightweight monitor config
• CLI option (waiting on feat: add fields via cli synthetics#963)
• 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: Port #4319 to serverless #4338

---

This is an automatic backport of pull request #4319 done by [Mergify](https://mergify.com).

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[colleenmcginnis]

🦭