Define `ecs.version` as a `constant_keyword` field

[axw]

We set ecs.version using a model processor, in the apm-server code, based on the version of the github.com/elastic/ecs/code/go/ecs module in use:

https://github.com/elastic/apm-server/blob/0c6c95ff8d6fb81a32ebf38420016b7d6ba765c2/internal/beater/processors.go#L64-L72

An older version of APM Server may send documents to the data streams, and the ingest pipeline is responsible for upgrading/transforming documents to the appropriate format. Therefore I believe it makes sense for the ECS version to be set by the installed ingest pipeline and index template. We can do this by making ecs.version a constant_keyword field.

This will also allow the code to be simplified, and documents to be made (slightly) smaller.

[simitt]

the ingest pipeline is responsible for upgrading/transforming documents to the appropriate format.

Would this issue also contain the work to put the ingest pipeline for adding the ECS version in place?

[axw]

Would this issue also contain the work to put the ingest pipeline for adding the ECS version in place?

I think all we need to do is:

• set ecs.version to a constant_keyword field in each data stream's ecs.yml file, and specify the constant value
• stop setting ecs.version in documents sent to Elasticsearch