Fully qualify Gitlab::Metrics to avoid code reloading issues

What does this MR do and why?

Sidekiq jobs can fail with Error fetching job: uninitialized constant Gitlab::Metrics if code is reloaded. Work around this issue by adding :: before Gitlab::Metrics.

Relates to #211875 (closed)

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.