Add note to deprecate usage_data.rb in CE and EE

ee/lib/ee/gitlab/usage_data.rb

 # frozen_string_literal: true
 
+# NOTE:
+# Implementing metrics direct in `usage_data.rb` is deprecated,
+# please add new instrumentation class and use add_metric method.
+# For more information, see https://docs.gitlab.com/ee/development/service_ping/metrics_instrumentation.html
+
 module EE
   module Gitlab
     module UsageData

lib/gitlab/usage_data.rb

@@ -10,6 +10,12 @@
 #   alt_usage_data { Gitlab::VERSION }
 #   redis_usage_data(Gitlab::UsageDataCounters::WikiPageCounter)
 #   redis_usage_data { ::Gitlab::UsageCounters::PodLogs.usage_totals[:total] }
+
+# NOTE:
+# Implementing metrics direct in `usage_data.rb` is deprecated,
+# please add new instrumentation class and use add_metric method.
+# For more information, see https://docs.gitlab.com/ee/development/service_ping/metrics_instrumentation.html
+
 module Gitlab
   class UsageData
     DEPRECATED_VALUE = -1000