code: def get_tracer(
          app: Application, tracer_key: str = APP_AIOZIPKIN_KEY) -> Tracer:
      
      return cast(Tracer, app[tracer_key])
docs: Returns tracer object from application context.
  
      By default tracer has APP_AIOZIPKIN_KEY in aiohttp application context,
      you can provide own key, if for some reason default one is not suitable.