Format wait time metric in dashboard

What does this MR do and why?

This change reduces the number of fraction digits in the queue duration displayed in the runner's dashboard.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. In rails console (rails c) enable the feature

   Feature.enable(:runners_dashboard)

2. (optional) Run bundle exec rake "gitlab:seed:runner_fleet" to generate a mock jobs.

3. Visit any the admin performance page http://gdk.test:3000/admin/runners/dashboard

   • No page links here yet, we will add that in later MRs.

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.

Related to #417260 (closed)