Given a tag and single run, return array of ScalarEvents.

def scalars_route(self, request):
    """Given a tag and single run, return array of ScalarEvents."""
    # TODO: return HTTP status code for malformed requests
    tag = request.args.get('tag')
    run = request.args.get('run')
    experiment = request.args.get('experiment')
    output_format = request.args.get('format')
    (body, mime_type) = self.scalars_impl(tag, run, experiment, output_format)
    return http_util.Respond(request, body, mime_type)