Unify metrics definition YAML file in one file

This is needed for data team in order to be easier to import the metric definitions.

This could be also seen as the Usage Ping schema

Proposal

Have a JSON unified file with all informations.

1. Have a rake task similar with the bundle exec rake gitlab:usage_data:generate_metrics_dictionary

Each time we change the definitions files the unified file should be updated.

2. Have an API for this where we return the unified data

cc @gitlab-org/growth/product-intelligence/engineers @mpeychet_