Extract process_name from GitLab::Sentry

What does this MR do?

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.

cc @ayufan see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24186#note_132000210

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24186
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21280

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer