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)
+}

models/metrics/label.go

@@ -0,0 +1,28 @@
+package metrics
+
+import (
+	"fmt"
+	"strings"
+)
+
+type Labels []Label
+
+type Label struct {
+	Key   string
+	Value string
+}
+
+func (l Labels) Print() string {
+	printedLabels := make([]string, len(l))
+	for i, e := range l {
+		printedLabels[i] = e.Print()
+	}
+	if len(l) == 0 {
+		return ""
+	}
+	return fmt.Sprintf("{%s}", strings.Join(printedLabels, ","))
+}
+
+func (l Label) Print() string {
+	return fmt.Sprintf("%s=\"%s\"", l.Key, l.Value)
+}

models/metrics/metric.go

@@ -0,0 +1,43 @@
+package metrics
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Hand-crafted Prometheus metrics
+// Since we're only using very simple counters in this application,
+// we don't actually need the official client SDK as a dependency
+
+type Metrics []Metric
+
+func (m Metrics) Print() (output string) {
+	printedMetrics := make(map[string]bool)
+	for _, m := range m {
+		if _, ok := printedMetrics[m.Key()]; !ok {
+			output += fmt.Sprintf("%s\n", m.Header())
+			printedMetrics[m.Key()] = true
+		}
+		output += fmt.Sprintf("%s\n", m.Print())
+	}
+
+	return output
+}
+
+func (m Metrics) Len() int {
+	return len(m)
+}
+
+func (m Metrics) Less(i, j int) bool {
+	return strings.Compare(m[i].Key(), m[j].Key()) < 0
+}
+
+func (m Metrics) Swap(i, j int) {
+	m[i], m[j] = m[j], m[i]
+}
+
+type Metric interface {
+	Key() string
+	Header() string
+	Print() string
+}