Define and describe metrics retiring process
Background
Recently there were two issues raised that was aimed to remove already existing keys in usage ping payload. During their implementation lack of defined process to retire some metric was identified. In order to provide cohesive behavior for all retired metrics, and in order to avoid retracing required steps whenever any other metric would get retired we need to define standard procedure to follow as it is done for adding new metric https://docs.gitlab.com/ee/development/product_analytics/usage_ping.html#developing-and-testing-usage-ping