metrics: listen on port 9090 for prometheus scrape

cmd/run.go

@@ -6,6 +6,7 @@ import (
 	"strings"
 
 	"github.com/metal-toolbox/flasher/internal/app"
+	"github.com/metal-toolbox/flasher/internal/metrics"
 	"github.com/metal-toolbox/flasher/internal/model"
 	"github.com/metal-toolbox/flasher/internal/store"
 	"github.com/metal-toolbox/flasher/internal/worker"
@@ -49,6 +50,9 @@ func runWorker(ctx context.Context) {
 		log.Fatal(err)
 	}
 
+	// serve metrics endpoint
+	metrics.ListenAndServe()
+
 	// Setup cancel context with cancel func.
 	ctx, cancelFunc := context.WithCancel(ctx)
 

internal/metrics/metrics.go

@@ -0,0 +1,29 @@
+package metrics
+
+import (
+	"log"
+	"net/http"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+const (
+	MetricsEndpoint = "0.0.0.0:9090"
+)
+
+// ListenAndServeMetrics exposes prometheus metrics as /metrics
+func ListenAndServe() {
+	go func() {
+		http.Handle("/metrics", promhttp.Handler())
+
+		server := &http.Server{
+			Addr:              MetricsEndpoint,
+			ReadHeaderTimeout: 2 * time.Second, // nolint:gomnd // time duration value is clear as is.
+		}
+
+		if err := server.ListenAndServe(); err != nil {
+			log.Println(err)
+		}
+	}()
+}