docs: def span(route):
      """Optional decorator for Flask routes.
  
      If you don't want to trace all routes using `Flask.before_request()' and 'Flask.after_request()'
      you can use this decorator as an alternative way to handle incoming B3 headers:
  
          @app.route('/instrumented')
          @span
          def instrumented():
              ...
              ...
              ...
  
      NB @span needs to come after (not before) @app.route.
      """
  
      @wraps(route)
      def route_decorator(*args, **kwargs):
          start_span()
          try:
              return route(*args, **kwargs)
          finally:
              end_span()
  
      return route_decorator