You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: rack_middleware.rb
    path of file: ./repos/gitlabhq/lib/gitlab/metrics
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

module Gitlab
  module Metrics
    # Rack middleware for tracking Rails and Grape requests.
    class RackMid
    
      def call(env)
        trans = Gitlab::Metrics::WebTransaction.new(env)

        begin
          retval = trans.run { @app.call(env) }

        rescue Exception => error # rubocop: disable Lint/RescueException
          trans.add_event(:rails_exception)

          raise error
        end