Given a single run, return the graph definition in protobuf format.

def graph_route(self, request):
    """Given a single run, return the graph definition in protobuf format."""
    run = request.args.get('run')
    tag = request.args.get('tag', '')
    conceptual_arg = request.args.get('conceptual', False)
    is_conceptual = True if conceptual_arg == 'true' else False

    if run is None:
      return http_util.Respond(
          request, 'query parameter "run" is required', 'text/plain', 400)

    limit_attr_size = request.args.get('limit_attr_size', None)
    if limit_attr_size is not None:
      try:
        limit_attr_size = int(limit_attr_size)
      except ValueError:
        return http_util.Respond(
            request, 'query parameter `limit_attr_size` must be an integer',
            'text/plain', 400)

    large_attrs_key = request.args.get('large_attrs_key', None)

    try:
      result = self.graph_impl(run, tag, is_conceptual, limit_attr_size, large_attrs_key)
    except ValueError as e:
      return http_util.Respond(request, e.message, 'text/plain', code=400)
    else:
      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)