Remove process_* metrics from gitlab-exporter

We're looking to sunset gitlab-exporter in #288043 (closed).

In Omnibus it is configured to export process_* metrics for the following services:

• sidekiq
• puma (gitlab-rails)
• unicorn
• git-upload-pack

I am basing this on the gitlab_exporter.yml template in Omnibus, but it could be configured differently by us or other users as it is just a template.

Our Helm chart does not configure a process block; I'm unclear on whether that means we use the a default process pattern or do not export process metrics at all.

Either way, the first 3 services already export process metrics as ruby_process_* via ruby_sampler, so those should be redundant.

Which leaves git-upload-pack, it is unclear if it is in use or where.

The aim of this issue is to:

• Find out what isn't already exported via other means
• Remove all gitlab-exporter process metrics if able to

🗒 After dropping the Omnibus configurations for the mentioned metrics, we could safely remove the code from gitlab-exporter, which is no longer needed.
The MR: https://gitlab.com/gitlab-org/gitlab-exporter/-/merge_requests/126