Add Sidekiq metrics for Gitaly and DB time

What does this MR do?

These are currently in our logs but not in Prometheus.

For gitlab-com/gl-infra/scalability#123 (closed).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.