Fix pid discovery for Unicorn in PidProvider

What does this MR do?

We'll not use ::Prometheus::Client::Support::Unicorn.worker_id in PidProvider, instead we'll parse $0 like we do for Puma.

The reason is ::Prometheus::Client::Support::Unicorn.worker_id provides incorrect value when is called from puma-master (expecting: nil, real: 0 or any worker was found with .first – see the related issue for details)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related to #64972 (closed)