[DOC] Clarify doc-level monitor `sample_documents` to preserve original source fields in action templates

[carmeroa]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request.
Please clarify in the Alerting documentation how sample_documents works for doc-level monitor action templates.

sample_documents should expose the original matched document with stable _id, _index, and _source fields so Mustache templates can safely reference original document fields such as {{_source.field_name}}. The docs should also mention that nested _source objects are preserved and that this is particularly useful for routed indices.

Version:
3.6

What other resources are available?

• Alerting issue: #2149
• Related PR: #2150
• Regression tests:
  • alerting/src/test/kotlin/org/opensearch/alerting/transport/DocLevelMonitorSampleDocumentsTests.kt

[kolchfa-aws]

Thank you for filing this issue. Since you have the subject matter expertise on this feature, could you please submit a pull request with the documentation changes? You can reference the existing alerting docs and add the clarifications about sample_documents behavior. We'll be happy to review it.