Add prometheus counter for error page responses

internal/staticpages/error_pages.go

@@ -7,8 +7,24 @@ import (
 	"path/filepath"
 
 	"gitlab.com/gitlab-org/gitlab-workhorse/internal/helper"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+var (
+	staticErrorResponses = prometheus.NewCounterVec(
+		prometheus.CounterOpts{
+			Name: "gitlab_workhorse_static_error_responses",
+			Help: "How many HTTP responses have been changed to a static error page, by HTTP status code.",
+		},
+		[]string{"code"},
+	)
 )
 
+func init() {
+	prometheus.MustRegister(staticErrorResponses)
+}
+
 type errorPageResponseWriter struct {
 	rw       http.ResponseWriter
 	status   int
@@ -43,6 +59,7 @@ func (s *errorPageResponseWriter) WriteHeader(status int) {
 		// check if custom error page exists, serve this page instead
 		if data, err := ioutil.ReadFile(errorPageFile); err == nil {
 			s.hijacked = true
+			staticErrorResponses.WithLabelValues(fmt.Sprintf("%d", s.status)).Inc()
 
 			helper.SetNoCacheHeaders(s.rw.Header())
 			s.rw.Header().Set("Content-Type", "text/html; charset=utf-8")