Fix startup crash in Puma single mode

What does this MR do and why?

Move Rails::SLI metrics init back in the initializer chain, into zz_metrics. Front-loading this in 7_prometheus caused dependency issues when running with a single Puma worker.

Fixes:

• omnibus-gitlab#6692 (closed)
• gitlab-compose-kit#62 (closed)

Screenshots or screen recordings

web_1               | Puma starting in single mode...
web_1               | * Puma version: 5.6.2 (ruby 2.7.5-p203) ("Birdie's Version")
web_1               | *  Min threads: 4
web_1               | *  Max threads: 4
web_1               | *  Environment: development
web_1               | *          PID: 7

How to set up and validate locally

1. Set puma workers to 0 ("single mode")
2. Start application
3. It should not crash in devise

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.