Add a environment injector for tracing Ruby process spawns

In LabKit Go, we pass tracing and correlation information from parent to client processes through the Environment Injector: https://gitlab.com/gitlab-org/labkit/blob/master/tracing/env_injector.go

We should add similar functionality for LabKit-Ruby.

Cross-reference https://gitlab.com/gitlab-org/gitlab-ce/issues/66545#note_208075061