Add the Response Content Length to Scout Context

content_length_middleware.rb

# Reports the response body content length to Scout as a "Response-Content-Length" context attribute on transactions. 
# This attribute is useful to correlate against slow requests that spend significant time in the view portion of a request,
# which is often related to rendering a large amount of data.
#
# To install in a Rails app:
# add to RAILS_ROOT/config/initializers/content_length_middleware.rb


# Rack::Sendfile is typically early in the middleware stack. 
# Want to ensure +ContentLengthMiddleware+ executes after +Rack::ContentLength+ returns.
Rails.application.config.middleware.use Rack::ContentLength # Adding this seems to prevent DevTrace from rendering.
Rails.application.config.middleware.insert_before Rack::ContentLength, "ScoutApm::ContentLengthMiddleware"

module ScoutApm
  class ContentLengthMiddleware

    CONTENT_LENGTH = 'Content-Length'.freeze

    def initialize(app)
      @app = app
    end

    def call(env)
      @status, @headers, @response = @app.call(env)
      if content_length
        ScoutApm::Context.add("Response-Content-Length" => content_length) # in bytes
      end
      [@status, @headers, @response]
    end

    private

    def content_length
      @headers[CONTENT_LENGTH]
    end
  end
end