[metrics] tracking Merge Requests picked into auto_deploy

What does this MR do and why?

How often do we pick into auto_deploy? How often does it fail?

I'm introducing a new metric (delivery_auto_deploy_picks_total) to keep track of the number of merge request processed by pick-into auto-deploy.

Show me the numbers 📈

The metric is now live. We can see it using thanos.