api_new_relic_instrumenter.rb

class ApiNewRelicInstrumenter < Grape::Middleware::Base
  include NewRelic::Agent::Instrumentation::ControllerInstrumentation

  def call_with_newrelic(&block)
    trace_options = {
      :category => :rack,
      :path => "#{route_path}\##{route_method}",
      :request => request
    }

    perform_action_with_newrelic_trace(trace_options) do
      result = yield
      MetricFrame.abort_transaction! if result.first == 404 # ignore cascaded calls
      result
    end
  end

  def call(env)
    @env = env
    if NewRelic::Agent.config[:monitor_mode]
      call_with_newrelic do
        super
      end
    else
      super
    end
  end

  def env
    @env
  end

  def route
    env['api.endpoint'].routes.first
  end

  def route_method
    route.route_method.downcase
  end

  def route_path
    path = route.route_path.gsub(/^.+:version\/|^\/|:|\(.+\)/, '').tr('/', '-')
    "api.#{route.route_version}.#{path}"
  end

  def response
    return if @app_response.nil?
    super
  end
end

The def response is needed. Described here: http://artsy.github.com/blog/2012/11/29/measuring-performance-in-grape-apis-with-new-relic/#comment-731239906. Not sure why. 204s?

Also, updated it to rely on the Agent config to determine whether it should run on line 20