Given a tag and single run, return array of histogram values.

def histograms_route(self, request):
    """Given a tag and single run, return array of histogram values."""
    tag = request.args.get('tag')
    run = request.args.get('run')
    try:
      (body, mime_type) = self.histograms_impl(
          tag, run, downsample_to=self.SAMPLE_SIZE)
      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)