Upgrade gitlab-labkit to 0.3.0
This matches the version used in Gitaly, bumps the gRPC version to be aligned with GitLab CE/EE, and adds gRPC interceptor support:
https://gitlab.com/gitlab-org/labkit-ruby/compare/v0.2.0...v0.3.0
EE port - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14178
Edited by Ash McKenzie