Pipeline creation duration metric

Description

We should have a metric showing pipeline creation duration. Ideally this should be Prometheus histogram.

Currently a pipeline creation process is really complex, and difficult to comprehend. It goes through numerous classes that do a lot of things, and we don't really know how long does it take to create a pipeline on average, on gitlab.com.

Benefits

Having this metric would allow us to optimize pipeline creation process, and identify problems before they become availability-related problems.