Add metrics

backend/mailgun/mailgun.go

@@ -5,9 +5,12 @@ import (
 	"io"
 	"io/ioutil"
 	"log"
+	"net/http"
 	"net/mail"
 
 	smtp "github.com/emersion/go-smtp"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 	mg "gopkg.in/mailgun/mailgun-go.v1"
 )
 
@@ -16,14 +19,16 @@ var _ smtp.User = &User{}
 
 // Backend type
 type Backend struct {
-	Domain     string
-	privateKey string
-	publicKey  string
+	Domain                 string
+	privateKey             string
+	publicKey              string
+	metricsMailgunMessages *prometheus.CounterVec
 }
 
 // User type
 type User struct {
-	mailgunClient mg.Mailgun
+	mailgunClient          mg.Mailgun
+	metricsMailgunMessages *prometheus.CounterVec
 }
 
 // NewBackend returns new instance of backend
@@ -32,28 +37,49 @@ func NewBackend(domain, privateKey, publicKey string) (smtp.Backend, error) {
 		return nil, fmt.Errorf("domain, privateKey, publicKey must not be empty")
 	}
 
-	return &Backend{
+	b := &Backend{
 		Domain:     domain,
 		privateKey: privateKey,
 		publicKey:  publicKey,
-	}, nil
+		metricsMailgunMessages: prometheus.NewCounterVec(
+			prometheus.CounterOpts{
+				Name: "mailgun_messages",
+				Help: "A counter for messages sent",
+			},
+			[]string{"status"},
+		),
+	}
+
+	prometheus.MustRegister(b.metricsMailgunMessages)
+
+	return b, nil
 }
 
 // Login is used to authenticate the user
 // In relay there's no need for that at the moment
 func (b *Backend) Login(username, password string) (smtp.User, error) {
 	return &User{
-		mailgunClient: mg.NewMailgun(b.Domain, b.privateKey, b.publicKey),
+		mailgunClient:          mg.NewMailgun(b.Domain, b.privateKey, b.publicKey),
+		metricsMailgunMessages: b.metricsMailgunMessages,
 	}, nil
 }
 
 // AnonymousLogin returns anonymouse user object
 func (b *Backend) AnonymousLogin() (smtp.User, error) {
 	return &User{
-		mailgunClient: mg.NewMailgun(b.Domain, b.privateKey, b.publicKey),
+		mailgunClient:          mg.NewMailgun(b.Domain, b.privateKey, b.publicKey),
+		metricsMailgunMessages: b.metricsMailgunMessages,
 	}, nil
 }
 
+func (b *Backend) ListenAndServeMetrics(addr string) error {
+	s := http.Server{
+		Addr:    addr,
+		Handler: promhttp.Handler(),
+	}
+	return s.ListenAndServe()
+}
+
 // Send will send email synchronously via Mailgun service
 func (u *User) Send(from string, to []string, r io.Reader) error {
 	m, err := mail.ReadMessage(r)
@@ -70,8 +96,10 @@ func (u *User) Send(from string, to []string, r io.Reader) error {
 		message := u.mailgunClient.NewMessage(from, m.Header.Get("Subject"), string(mBody), recipient)
 		resp, id, err := u.mailgunClient.Send(message)
 		if err != nil {
+			u.metricsMailgunMessages.WithLabelValues("fail").Inc()
 			return err
 		}
+		u.metricsMailgunMessages.WithLabelValues("success").Inc()
 		log.Printf("ID: %s Resp: %s", id, resp)
 	}
 	return nil

relay.go

@@ -19,6 +19,7 @@ type config struct {
 	MaxIdleSeconds    int    `default:"300" envconfig:"MAX_IDLE_SECONDS"`
 	MaxMessageBytes   int    `default:"1048576" envconfig:"MAX_MESSAGE_BYTES"`
 	MaxRecipients     int    `default:"50" envconfig:"MAX_RECIPIENTS"`
+	MetricsAddr       string `default:":8081" envconfig:"METRICS_ADDR"`
 }
 
 func main() {
@@ -47,6 +48,10 @@ func main() {
 	s.AuthDisabled = cfg.AuthDisabled
 	s.AllowInsecureAuth = cfg.AllowInsecureAuth
 
+	go func(metricsAddr string) {
+		log.Fatal(be.(*mailgun.Backend).ListenAndServeMetrics(metricsAddr))
+	}(cfg.MetricsAddr)
+
 	log.Println("Starting server at", s.Addr)
 	if err := s.ListenAndServe(); err != nil {
 		log.Fatal(err)