feat(core) add X-Kong-Upstream-Status header

While Kong returns the same status code as
that of the upstream when it proxies a request,
it is useful to record the real status code
returned by the upstream.

This can be used for monitoring Kong
itself if it is returning the same response
as the upstream.

kong/constants.lua

@@ -41,6 +41,7 @@ return {
     HOST_OVERRIDE = "X-Host-Override",
     PROXY_LATENCY = "X-Kong-Proxy-Latency",
     UPSTREAM_LATENCY = "X-Kong-Upstream-Latency",
+	UPSTREAM_STATUS = "X-Kong-Upstream-Status",
     CONSUMER_ID = "X-Consumer-ID",
     CONSUMER_CUSTOM_ID = "X-Consumer-Custom-ID",
     CONSUMER_USERNAME = "X-Consumer-Username",

kong/core/handler.lua

@@ -523,6 +523,7 @@ return {
         local now = get_now()
         ctx.KONG_WAITING_TIME = now - ctx.KONG_ACCESS_ENDED_AT -- time spent waiting for a response from upstream
         ctx.KONG_HEADER_FILTER_STARTED_AT = now
+		header[constants.HEADERS.UPSTREAM_STATUS]  = ngx.status
       end
     end,
     after = function(ctx)