expose job url in metrics

What does this MR do?

It adds another field in gitlab_runner_jobs metric named joburl which is the url of the job being executed.

Why was this MR needed?

It gives better visibility at what job is currently being executed by the given runner.

What's the best way to test this MR?

Once the runner is running, do a curl request to metrics endpoint curl -s "http://localhost:9252/metrics" | grep -i 'gitlab_runner_job' It should return something like this

What are the relevant issue numbers?