Count senddata responses in prometheus

internal/senddata/injecter.go

@@ -10,6 +10,7 @@ import (
 type Injecter interface {
 	Match(string) bool
 	Inject(http.ResponseWriter, *http.Request, string)
+	Name() string
 }
 
 type Prefix string
@@ -30,3 +31,7 @@ func (p Prefix) Unpack(result interface{}, sendData string) error {
 	}
 	return nil
 }
+
+func (p Prefix) Name() string {
+	return strings.TrimSuffix(string(p), ":")
+}

internal/senddata/senddata.go

@@ -4,8 +4,24 @@ import (
 	"net/http"
 
 	"gitlab.com/gitlab-org/gitlab-workhorse/internal/helper"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+var (
+	sendDataResponses = prometheus.NewCounterVec(
+		prometheus.CounterOpts{
+			Name: "gitlab_workhorse_senddata_responses",
+			Help: "How many HTTP responses have been hijacked by a workhorse senddata injecter",
+		},
+		[]string{"name"},
+	)
 )
 
+func init() {
+	prometheus.MustRegister(sendDataResponses)
+}
+
 type sendDataResponseWriter struct {
 	rw        http.ResponseWriter
 	status    int
@@ -64,6 +80,7 @@ func (s *sendDataResponseWriter) tryInject() bool {
 	for _, injecter := range s.injecters {
 		if injecter.Match(header) {
 			s.hijacked = true
+			sendDataResponses.WithLabelValues(injecter.Name()).Inc()
 			helper.DisableResponseBuffering(s.rw)
 			injecter.Inject(s.rw, s.req, header)
 			return true