set ENV['prometheus_multiproc_dir'] in config/boot.rb instead of config.ru

This MR makes the default 'prometheus_multiproc_dir' env variable available from all three contexts - sidekiq, rails c and rails s

Why was this MR needed?

when using prometheus metrics from sidekiq or console in development environment Prometheus client threw exceptions.

Does this MR meet the acceptance criteria?

=- Tests

• All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

Related to: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2118