[#175] Add health metric

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

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))
 }