[#451] metrics: Move to internal

`metrics` don't look like something others want to import.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

internal/metrics/state.go

@@ -0,0 +1,29 @@
+package metrics
+
+import (
+	"git.frostfs.info/TrueCloudLab/frostfs-observability/metrics"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+type StateMetrics interface {
+	SetHealth(s int32)
+}
+
+type stateMetrics struct {
+	healthCheck prometheus.Gauge
+}
+
+func newStateMetrics() *stateMetrics {
+	return &stateMetrics{
+		healthCheck: metrics.NewGauge(prometheus.GaugeOpts{
+			Namespace: namespace,
+			Subsystem: stateSubsystem,
+			Name:      "health",
+			Help:      "Current Node state",
+		}),
+	}
+}
+
+func (m *stateMetrics) SetHealth(s int32) {
+	m.healthCheck.Set(float64(s))
+}