http://verboselogging.com/2013/07/29/newrelic-sinatra-rails-metal-controllers

newrelic.rb

require 'new_relic/agent/instrumentation/sinatra'

module NewRelic
  module Agent
    module Instrumentation
      module Sinatra
        def ignore_apdex?
          skip = self.response.status.to_i == 404
          if skip
            env.delete('sinatra.error')
            NewRelic::Agent::Transaction.current.exceptions.clear
          end
          skip
        end
      end
    end
  end
end