Add gitlab-metrics-exporter component

As part of gitlab-metrics-exporter#3 (closed), we need to put gitlab-metrics-exporter on PATH so that Sidekiq and Puma can start the process.

This should happen transparently to users, so I added the repository to all build targets; the rails startup script then installs the exporter into /usr/local. It compiles very quickly so I think it's OK to do this every time.

Note:

• The exporter does not run by default. You must set GITLAB_GOLANG_METRICS_SERVER=1 until it is considered production ready.
• The actual integration with the Rails app is happening in gitlab!84567 (merged), but these MRs need not block each other.
• This will require a make build and make update-repos on the first checkout.