Use a more standardized project layout

In #2, mrngm made the point that we should move to a more
standardized project structure. This commit does exactly that.
The new project structure is based on the repository listed at
<https://github.com/golang-standards/project-layout>.

Fixes #2.

internal/metrics/metrics.go

@@ -0,0 +1,55 @@
+package metrics
+
+import (
+	"log"
+	"net/http"
+	"time"
+
+	"github.com/gorilla/mux"
+	"github.com/pkg/errors"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+	bolt "go.etcd.io/bbolt"
+
+	"gitea.hashru.nl/dsprenkels/rushlink/internal/db"
+)
+
+const (
+	METRICS_ADDR = "127.0.0.1:58614"
+)
+
+func StartMetricsServer() {
+	var (
+		_ = promauto.NewGaugeFunc(prometheus.GaugeOpts{
+			Namespace: "rushlink",
+			Subsystem: "shorten",
+			Name:      "urls_total",
+			Help:      "The current amount of shortened urls in the database.",
+		}, func() float64 {
+			var metric float64
+			if err := db.DB.View(func(tx *bolt.Tx) error {
+				bucket := tx.Bucket([]byte("shorten"))
+				if bucket == nil {
+					return errors.New("bucket 'shorten' could not be found")
+				}
+				metric = float64(bucket.Stats().KeyN)
+				return nil
+			}); err != nil {
+				log.Printf("error: %v", errors.Wrap(err, "fetching shorten_urls_total metric"))
+				return 0
+			}
+			return metric
+		})
+	)
+
+	router := mux.NewRouter()
+	router.Handle("/metrics", promhttp.Handler()).Methods("GET")
+	srv := &http.Server{
+		Handler:      router,
+		Addr:         METRICS_ADDR,
+		WriteTimeout: 15 * time.Second,
+		ReadTimeout:  15 * time.Second,
+	}
+	log.Fatal(srv.ListenAndServe())
+}