Given a tag and a run, return the session.run() metadata.

def run_metadata_route(self, request):
    """Given a tag and a run, return the session.run() metadata."""
    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)