This enables us to calculate the cache hit ratio, which may help
determine whether more caching would be beneficial.

Please note that this only counts requests for which caching is enabled
(e.g. {fastcgi,proxy}_cache is configured), e.g. for static served files
cache_status will be "".

[1] http://nginx.org/en/docs/http/ngx_http_upstream_module.html#var_upstream_cache_status