metrics: create 3scale backend status

Author: davidor

gateway/src/apicast/metrics/3scale_backend_status.lua

@@ -0,0 +1,25 @@
+local prometheus = require('apicast.prometheus')
+local metrics_updater = require('apicast.metrics.updater')
+
+local format = string.format
+
+local _M = {}
+
+local backend_response_metric = prometheus(
+  'counter',
+  'backend_response',
+  "Response status codes from 3scale's backend",
+  { 'status' }
+)
+
+function _M.inc(status)
+  if not status or status == 0 then
+    ngx.log(ngx.WARN, 'Invalid status received')
+  end
+
+  local label = format("%dxx", status/100)
+
+  metrics_updater.inc(backend_response_metric, label)
+end
+
+return _M