[#175] Add health metric

Signed-off-by: Denis Kirillov <denis@nspcc.ru>

app.go

@@ -31,6 +31,7 @@ type (
 		webServer *fasthttp.Server
 		webDone   chan struct{}
 		resolver  *resolver.ContainerResolver
+		metrics   GateMetricsProvider
 	}
 
 	// App is an interface for the main gateway function.
@@ -41,6 +42,10 @@ type (
 
 	// Option is an application option.
 	Option func(a *app)
+
+	GateMetricsProvider interface {
+		SetHealth(int32)
+	}
 )
 
 // WithLogger returns Option to set a specific logger.
@@ -151,6 +156,10 @@ func newApp(ctx context.Context, opt ...Option) App {
 		a.log.Info("container resolver is disabled")
 	}
 
+	if a.cfg.GetBool(cmdMetrics) {
+		a.metrics = newGateMetrics()
+	}
+
 	return a
 }
 
@@ -231,6 +240,10 @@ func getKeyFromWallet(w *wallet.Wallet, addrStr string, password *string) (*ecds
 
 func (a *app) Wait() {
 	a.log.Info("starting application", zap.String("app_name", "neofs-http-gw"), zap.String("version", Version))
+	if a.metrics != nil {
+		a.metrics.SetHealth(1)
+	}
+
 	<-a.webDone // wait for web-server to be stopped
 }
 

metrics.go

@@ -9,6 +9,47 @@ import (
 	"go.uber.org/zap"
 )
 
+const (
+	namespace      = "neofs_http_gw"
+	stateSubsystem = "state"
+)
+
+type GateMetrics struct {
+	stateMetrics
+}
+
+type stateMetrics struct {
+	healthCheck prometheus.Gauge
+}
+
+func newGateMetrics() *GateMetrics {
+	stateMetric := newStateMetrics()
+	stateMetric.register()
+
+	return &GateMetrics{
+		stateMetrics: stateMetric,
+	}
+}
+
+func newStateMetrics() stateMetrics {
+	return stateMetrics{
+		healthCheck: prometheus.NewGauge(prometheus.GaugeOpts{
+			Namespace: namespace,
+			Subsystem: stateSubsystem,
+			Name:      "health",
+			Help:      "Current HTTP gateway state",
+		}),
+	}
+}
+
+func (m stateMetrics) register() {
+	prometheus.MustRegister(m.healthCheck)
+}
+
+func (m stateMetrics) SetHealth(s int32) {
+	m.healthCheck.Set(float64(s))
+}
+
 func attachMetrics(r *router.Router, l *zap.Logger) {
 	r.GET("/metrics/", metricsHandler(prometheus.DefaultGatherer, l))
 }