Document @metadata fields

[lucabelluccini]

Describe the enhancement:

We document @metadata._id field in the scope of deduplication of events.

We recently added @metadata.op_type in #12606 to allow other operations, such as index instead of create (default).

It would be nice to add a dedicated documentation page on how to use them. E.g.

processors:
  - script:
      lang: javascript
      id: my_filter
      source: >
        function process(event) {
            event.Put("@metadata.op_type", "index");
        }

[elasticmachine]

Pinging @elastic/obs-docs (Team:Docs)

[botelastic]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.