Clarifying metric descriptions

Overview

This MR is a result of a Slack thread regarding the definitions of some usage ping metrics. The goal of the MR is to clarify the metrics' descriptions to map more closely to what they are actually counting. Please let me know if more information is needed, thanks!

usage_activity_by_stage_monthly.secure.user_api_fuzzing_jobs

We are counting distinct user_id from ci_builds for name apifuzzer_fuzz

usage_activity_by_stage.create.projects_with_repositories_enabled

Count of distinct creator_id from projects joined with project_features where repository_access_level = 20 for all time.