Add nested type support to go code generator

[ebeahan]

Introducing type: nested will require handling of nested fields be defined, else the go code generator will fail.

[ebeahan]

Adjusted this PR to add both nested as well as flattened types to the generator's mapping logic.

[marc-gr]

Is it possible to add some test cases un the tests folder for this addition?

[andrewstucki]

@ebeahan @marc-gr I know that there is a plan to eventually strip out the go generators from the ECS repo, does it make sense to do so now? I believe that the only elastic project using the go code is packetbeat, and I think it's pinned to an older version. Otherwise, if we're planning on keeping these around for awhile, maybe we could add the binary type, making it []byte would allow the default json marshaler to automatically do base64 encoding.

[ebeahan]

@andrewstucki @marc-gr Thanks for taking a look. These are some good discussion points, but I don't want to expand the original scope here too much. Any objections to the proposed mapping for nested and flattened?

Is it possible to add some test cases un the tests folder for this addition?

Ideally, we'd have something in place for testing the go generator portions of the repo, but we don't today. I don't think we should add that testing framework into the ECS repo right now since...

I know that there is a plan to eventually strip out the go generators from the ECS repo, does it make sense to do so now?

...we definitely want to remove the go generator and eventually the other golang dependencies from this repo.

I have no issue with pulling it out, but I'm also not familiar with where it's a dependency. A team or individual would need to take ownership of setting up the new repo; I don't see this as something the ECS team would be best to maintain (right now at least).

[marc-gr]

Ideally, we'd have something in place for testing the go generator portions of the repo, but we don't today. I don't think we should add that testing framework into the ECS repo right now

👍

[ebeahan]

Will focus on solely on adding nested type support here. Separate PR opened to add support for flattened: #1302.

[ebeahan]

Took a shot at the improved implementation suggested in #1254 (comment).

@andrewkroh @andrewstucki would appreciate a look.

[andrewkroh]

I think this is good, but I would find it very helpful if you could temporarily add some nested fields to demonstrate what the generated code looks like and then paste that into comment.

[ebeahan]

I generated some examples using some nested fields from the experimental schema: https://gist.github.com/ebeahan/2c3038b0accd1786f78d7d7f0b4353da

[andrewkroh]

Thanks! It looks like the tag is missing the key name. It has

	// ELF object segment list.
	Segments []Segments `ecs:""`

where I expected

Segments []Segments `ecs:"segments"`

[ebeahan]

Thanks, @andrewkroh. I believe it's now addressed.

Updated the sample gist with the latest versions.