Add docs for track_redis_events helper methods in controllers

With !39796 (merged) we bring helper method to track events using Redis HLL

track_redis_hll_event :index, name: 'i_analytics_dev_ops_score', feature: :my_feature

Add this option to Telemetry guide https://docs.gitlab.com/ee/development/telemetry/usage_ping.html#redis-hll-counters