Adding event.transaction.id

[vbohata]

For some events I need to track their transaction/session or correlation ids. I think there should be a new field event.transaction.id for this use case.

[ruflin]

I think we can take inspiration here from apm: https://github.com/elastic/apm-server/blob/master/_meta/fields.common.yml#L266

Here it is transaction.id, I would not nest it under event.

[ebeahan]

ECS now contains the transaction.id field.

There's some recent conversation taking place around introducing a session.id field in the session RFC to associate multiple events together based on a common session identifier.