Add runner managers count to runner details

What does this MR do and why?

This is a small iteration towards -> #409388 (closed)

Add runner managers count to runner details

This change adds the number of runner managers to the runner details when available.

Changelog: added

Screenshots or screen recordings

How to set up and validate locally

You may see 0 or 1 on most test runners you use, to test this fully we can simulate having two separate machines with runner installed.

Install runner locally

1. brew install runner works on MacOS

Follow the instructions to create a runner

1. Admin -> Runners
2. Select "Create instance runner"
3. Ensure you run the gitlab-runner register ... step

Simulate two machines

1. Duplicate the configuration files cp -R ~/.gitlab-runner ~/.gitlab-runner-2
2. Edit the random system id open ~/.gitlab-runner-2/.runner_system_id (changing one character will do)
3. Run runner 1 gitlab-runner run
   • the default configuration at ~/.gitlab-runner will be used
4. In another console tab, run runner 2 gitlab-runner run --config ~/.gitlab-runner-2/config.toml

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 #409388 (closed)