Auto-populate "introduced_by_url"

Related to gitlab-org/gitlab#351471 (closed)

Automatically assign "introduced_by_url" for metrics that don't have this attribute.

Steps to replicate locally

1. run bin/build
2. wait. The metrics fetching can take even two hours
3. Run yarn --cwd ./web dev, open the metrics dictionary in a web browser and check if the urls have been auto-loaded for metrics don't include them in their yml files.