Record upstream failed connections

[maneta]

We need to record failed connections to the upstream. The idea is to log those upstream calls so we could detect failing backends.

• The variable $upstream_status should have the upstream response code ([prometheus] metric for upstream status / proxy status #30)
• The variables $upstream_connect_time and $upstream_response_time are usefull to debug timeouts and $upstream_addr should have the resolved ip address we tried to connect to.

[mikz]

Also it is possible to detect if the failure comes from openresty. If there is some error ngx.status will be 500, but $upstream_status will be nil.