Add Apache headers to all .go source files

[andrewkroh]

Headers were added by https://github.com/elastic/go-licenser.

Make Changes

Added Targets

• check-headers - verifies headers exist on .go files.
• add-headers - adds headers to all .go files.

Changed Targets

• check - added dependency on check-headers.
• fmt - added dependency on add-headers.

Variables

• CHECK_HEADERS_DISABLED - if set then headers are neither checked nor added.

REVIEWERS: I separated this into two commits to make reviewing simpler. You can squash them together.

• c5cd1f5 - This has the tooling changes.
• 156763f - This has the updates to the .go files to add headers.

[andrewkroh]

I opened some issues with go-licenser to allow for better integration. Specififically when we add Go x-pack content we will need a way to check those licenses.

• Add --exclude flag go-licenser#10
• Support Elastic License as well as Apache 2.0 go-licenser#1

[ruflin]

@andrewkroh Is there an automated way to add the headers to all files? How did you add them?

[andrewkroh]

The same tool that checks the headers can be used to add them.

go-licenser - This adds headers to all .go files recursively.
go-licenser -d - Checks for the existence of headers.

[andrewkroh]

As discussed with @ruflin and @urso , make fmt will add headers to .go files.

[ruflin]

PR looks good to me and we should merge it rather soonish. I think we should have an entry at least in the developer changelog for this.

[ph]

Changes LGTM, @andrewkroh won the commit race for 2018. :)

I think we want to apply the same logic to other source code, like our python tests? (in another PR of course)

edited s/wont/won.. thank you phone.