_appsignal.html.erb

<% if ENV['APPSIGNAL_FRONTEND_API_KEY'].present? %>
  <script type="application/javascript">
    window.appsignalConfig = {
      frontendApiKey: "<%= ENV.fetch('APPSIGNAL_FRONTEND_API_KEY') %>"
    };
  </script>

  <%= javascript_include_tag 'appsignal' %>
<% end %>