[#1614] metrics: Add health metrics

Also, rename metrics structure since it collects not only storage metrics
now.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

pkg/metrics/state.go

@@ -0,0 +1,28 @@
+package metrics
+
+import "github.com/prometheus/client_golang/prometheus"
+
+const stateSubsystem = "state"
+
+type stateMetrics struct {
+	healthCheck prometheus.Gauge
+}
+
+func newStateMetrics() stateMetrics {
+	return stateMetrics{
+		healthCheck: prometheus.NewGauge(prometheus.GaugeOpts{
+			Namespace: namespace,
+			Subsystem: stateSubsystem,
+			Name:      "health",
+			Help:      "Current Node state",
+		}),
+	}
+}
+
+func (m stateMetrics) register() {
+	prometheus.MustRegister(m.healthCheck)
+}
+
+func (m stateMetrics) SetHealth(s int32) {
+	m.healthCheck.Set(float64(s))
+}