fix: prometheus metrics types

models/metrics/gauge_metric.go

@@ -0,0 +1,22 @@
+package metrics
+
+import "fmt"
+
+type GaugeMetric struct {
+	Name   string
+	Value  int64
+	Desc   string
+	Labels Labels
+}
+
+func (c GaugeMetric) Key() string {
+	return c.Name
+}
+
+func (c GaugeMetric) Print() string {
+	return fmt.Sprintf("%s%s %d", c.Name, c.Labels.Print(), c.Value)
+}
+
+func (c GaugeMetric) Header() string {
+	return fmt.Sprintf("# HELP %s %s\n# TYPE %s gauge", c.Name, c.Desc, c.Name)
+}