Vendor in Metrics buildpack When `barnes` gem is detected

To avoid issues with pipeline promotion and to remove one step from enabling Ruby metrics, we can have the Ruby buildpack vendor in the metrics buildpack:

https://github.com/heroku-buildpacks/heroku-buildpack-metrics

Start by reading the Ruby buildpack repository and the linked heroku-buildpack-metrics project, focusing on how the barnes gem is detected and how buildpack dependencies are supplied. The change is complete when the metrics buildpack is vendored automatically for apps where barnes is detected, avoiding a separate pipeline-promotion step.

Written by the indexing model from the issue text.