Make trigger variables available from API/Webhook
Description
Trigger variables are able to make marginal change for the build/pipeline result, and can make every build different. The problem is, that there are no solution to get these values in a programmatically way, therefore these important differences remain hidden. In other words, there is no programmatical way to track back the exact inputs for a build, so it cannot be identified.
All the APIs regarding to variables bound to projects, and not to builds. This means the builds are not differentiated inside the project. https://docs.gitlab.com/ce/api/build_variables.html
The UI has the feature to get these values but the APIs have not. https://docs.gitlab.com/ce/ci/triggers/README.html#pass-build-variables-to-a-trigger
The current workaround solution is to invoke a custom webhook (curl or python request) as a step of the build which reports back the trigger values to the triggerer along with the build Id.
Proposal
Make these trigger variables available via the webhook and/or API.