feat: add :51090/metrics endpoint with Prometheus metrics listening on machine IP (#304)

Signed-off-by: Miek Gieben <miek@miek.nl>
Co-authored-by: Pasha Sviderski <me@psviderski.name>

internal/metrics/metrics.go

@@ -0,0 +1,35 @@
+// Package metrics declares all metrics Uncloud uses.
+package metrics
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var (
+	Version = promauto.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: Namespace, Subsystem: "uncloudd",
+		Name: "build_info",
+		Help: "Build information.",
+	}, []string{"version"})
+	DNSQuery = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: Namespace, Subsystem: "dns",
+		Name: "query_total",
+		Help: "Counter of DNS queries.",
+	}, []string{"internal", "status"})
+)
+
+const (
+	Err = "err"
+	Ok  = "ok"
+)
+
+// Status returns "ok" if err is nil, otherwise "err".
+func Status(err error) string {
+	if err != nil {
+		return Err
+	}
+	return Ok
+}
+
+const Namespace = "uncloud"