code: def run_metadata_route(self, request):
      
      tag = request.args.get('tag')
      run = request.args.get('run')
      if tag is None:
        return http_util.Respond(
            request, 'query parameter "tag" is required', 'text/plain', 400)
      if run is None:
        return http_util.Respond(
            request, 'query parameter "run" is required', 'text/plain', 400)
      result = self.run_metadata_impl(run, tag)
      if result is not None:
        (body, mime_type) = result  # pylint: disable=unpacking-non-sequence
        return http_util.Respond(request, body, mime_type)
      else:
        return http_util.Respond(request, '404 Not Found', 'text/plain',
                                 code=404)
docs: Given a tag and a run, return the session.run() metadata.