metricsserver: add helper for exporting prom metrics

(also from the monitor code)

metricsserver/metrics.go

@@ -0,0 +1,68 @@
+package metricsserver
+
+import (
+	"context"
+	"net/http"
+	"strconv"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+
+	"go.ntppool.org/common/logger"
+)
+
+type Metrics struct {
+	r *prometheus.Registry
+}
+
+func New() *Metrics {
+	r := prometheus.NewRegistry()
+
+	m := &Metrics{
+		r: r,
+	}
+
+	return m
+}
+
+func (m *Metrics) Registry() prometheus.Registerer {
+	return m.r
+}
+
+func (m *Metrics) Handler() http.Handler {
+
+	log := logger.NewStdLog("prom http", false, nil)
+
+	return promhttp.HandlerFor(m.r, promhttp.HandlerOpts{
+		ErrorLog:          log,
+		Registry:          m.r,
+		EnableOpenMetrics: true,
+	})
+}
+
+func (m *Metrics) ListenAndServe(ctx context.Context, port int) {
+
+	log := logger.Setup()
+
+	srv := &http.Server{
+		Addr:         ":" + strconv.Itoa(port),
+		ReadTimeout:  10 * time.Second,
+		WriteTimeout: 20 * time.Second,
+		IdleTimeout:  120 * time.Second,
+		Handler:      m.Handler(),
+	}
+
+	go func() {
+		err := srv.ListenAndServe()
+		if err != http.ErrServerClosed {
+			log.Warn("metrics server done listening", "err", err)
+		}
+	}()
+
+	<-ctx.Done()
+
+	if err := srv.Shutdown(ctx); err != nil {
+		log.Error("metrics server shutdown failed", "err", err)
+	}
+}