Flipper comes with automatic instrumentation. By default these work with ActiveSupport::Notifications, but only require the pieces of ActiveSupport that are needed and only do so if you actually attempt to require the instrumentation files listed below.
To use the log subscriber:
# Gemfile
gem "activesupport"
# config/initializers/flipper.rb (or wherever you want it)
require "flipper/instrumentation/log_subscriber"
To use the statsd instrumentation:
# Gemfile
gem "activesupport"
gem "statsd-ruby"
# config/initializers/flipper.rb (or wherever you want it)
require "flipper/instrumentation/statsd"
Flipper::Instrumentation::StatsdSubscriber.client = Statsd.new # or whatever your statsd instance is
You can also do whatever you want with the instrumented events. Check out this example for more.