feat: add prometheus metrics without external standalone exporter

models/metrics/counter_metric.go

@@ -0,0 +1,22 @@
+package metrics
+
+import "fmt"
+
+type CounterMetric struct {
+	Name   string
+	Value  int
+	Desc   string
+	Labels Labels
+}
+
+func (c CounterMetric) Key() string {
+	return c.Name
+}
+
+func (c CounterMetric) Print() string {
+	return fmt.Sprintf("%s%s %d", c.Name, c.Labels.Print(), c.Value)
+}
+
+func (c CounterMetric) Header() string {
+	return fmt.Sprintf("# HELP %s %s\n# TYPE %s counter", c.Name, c.Desc, c.Name)
+}