NewRelic tracks your app and provides info and graphs about throughput, slow requests, errors, etc.
Follow this workflow to set it up for your Rails app. You will need an account, get one.

1. Add the gem to gemfile, bundle

group :production, :staging do
  gem "newrelic_rpm"
end

2. Get a clean newrelic.yml from website

Given account main view at https://rpm.newrelic.com/accounts/123,
your yml should be at https://rpm.newrelic.com/accounts/123/newrelic.yml

Use your account id number or search for “newrelic.yml” in the main view.

3. Place the fresh yml in apps config/

4. Edit the yaml (change name etc)