Add metric to count whether enabling BBR succeeded (#363)

* Add metric to count whether bbr enabled successfully

ndt7/measurer/measurer.go

@@ -7,6 +7,8 @@ import (
 	"time"
 
 	"github.com/gorilla/websocket"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
 
 	"github.com/m-lab/go/memoryless"
 	"github.com/m-lab/ndt-server/logging"
@@ -15,6 +17,16 @@ import (
 	"github.com/m-lab/ndt-server/netx"
 )
 
+var (
+	BBREnabled = promauto.NewCounterVec(
+		prometheus.CounterOpts{
+			Name: "ndt7_measurer_bbr_enabled",
+			Help: "A counter of every attempt to enable bbr.",
+		},
+		[]string{"status"},
+	)
+)
+
 // Measurer performs measurements
 type Measurer struct {
 	conn   *websocket.Conn
@@ -33,10 +45,13 @@ func New(conn *websocket.Conn, UUID string) *Measurer {
 func (m *Measurer) getSocketAndPossiblyEnableBBR() (netx.ConnInfo, error) {
 	ci := netx.ToConnInfo(m.conn.UnderlyingConn())
 	err := ci.EnableBBR()
+	success := "true"
 	if err != nil {
+		success = "false"
 		logging.Logger.WithError(err).Warn("Cannot enable BBR")
 		// FALLTHROUGH
 	}
+	BBREnabled.WithLabelValues(success).Inc()
 	return ci, nil
 }