GitLab::Sentry has a program_context method to determine whether a
Sentry exception occurred in Sidekiq or rails. Since we will need
similar functionality for distributed tracing, this change extracts the
program_context method into GitLab.process_name for more general
consumption.
The Correlation ID is taken or generated from received X-Request-ID.
Then it is being passed to all executed services (sidekiq workers
or gitaly calls).
The Correlation ID is logged in all structured logs as `correlation_id`.
The initializers including this were doing so at the top level, so every object
loaded after them had a `current_application_settings` method. However, if
someone had rack-attack enabled (which was loaded before these initializers), it
would try to load the API, and fail, because `Gitlab::CurrentSettings` didn't
have that method.
To fix this:
1. Don't include `Gitlab::CurrentSettings` at the top level. We do not need
`Object.new.current_application_settings` to work.
2. Make `Gitlab::CurrentSettings` explicitly `extend self`, as we already use it
like that in several places.
3. Change the initializers to use that new form.
