feat: add getting visitor counter

internal/service/visitor.go

@@ -0,0 +1,30 @@
+package service
+
+import (
+	"encoding/json"
+	"io"
+	"log/slog"
+	"os"
+	"path/filepath"
+
+	"backend/internal/model"
+)
+
+func GetVisitorCounter() model.VisitorCounter {
+	path := filepath.Join(".", "data", "visitor.json")
+	jsonFile, err := os.Open(path)
+	if err != nil {
+		slog.Warn("File not found!", slog.Any("file", path))
+		return model.VisitorCounter{}
+	}
+
+	bytes, _ := io.ReadAll(jsonFile)
+
+	var data model.VisitorCounter
+	err = json.Unmarshal(bytes, &data)
+	if err != nil {
+		slog.Error("Error unmarshalling JSON", slog.Any("error", err), slog.Any("file", path))
+	}
+
+	return data
+}