Given a tag and single run, return an array of compressed histograms.

def distributions_route(self, request):
    """Given a tag and single run, return an array of compressed histograms."""
    tag = request.args.get('tag')
    run = request.args.get('run')
    try:
      (body, mime_type) = self.distributions_impl(tag, run)
      code = 200
    except ValueError as e:
      (body, mime_type) = (str(e), 'text/plain')
      code = 400
    return http_util.Respond(request, body, mime_type, code=code)