feat: add prometheus metrics

backend/internal/middleware/metrics.go

@@ -0,0 +1,108 @@
+package middleware
+
+import (
+	"net/http"
+	"strconv"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+type PrometheusMetricsMiddleware struct {
+	reqCnt       *prometheus.CounterVec
+	reqDur       *prometheus.HistogramVec
+	reqSz, resSz prometheus.Summary
+}
+
+func computeApproximateRequestSize(r *http.Request) int {
+	s := 0
+	if r.URL != nil {
+		s = len(r.URL.Path)
+	}
+
+	s += len(r.Method)
+	s += len(r.Proto)
+	for name, values := range r.Header {
+		s += len(name)
+		for _, value := range values {
+			s += len(value)
+		}
+	}
+	s += len(r.Host)
+
+	if r.ContentLength != -1 {
+		s += int(r.ContentLength)
+	}
+	return s
+}
+
+func NewPrometheusMetricsMiddleware() *PrometheusMetricsMiddleware {
+
+	m := &PrometheusMetricsMiddleware{}
+
+	m.reqCnt = prometheus.NewCounterVec(
+		prometheus.CounterOpts{
+			Subsystem: "pocket_id",
+			Name:      "requests_total",
+			Help:      "How many HTTP requests processed, partitioned by status code and HTTP method.",
+		},
+		[]string{"code", "method", "handler", "host", "url"},
+	)
+
+	m.reqDur = prometheus.NewHistogramVec(
+		prometheus.HistogramOpts{
+			Subsystem: "pocket_id",
+			Name:      "request_duration_seconds",
+			Help:      "The HTTP request latencies in seconds.",
+		},
+		[]string{"code", "method", "url"},
+	)
+
+	m.resSz = prometheus.NewSummary(
+		prometheus.SummaryOpts{
+			Subsystem: "pocket_id",
+			Name:      "response_size_bytes",
+			Help:      "The HTTP response sizes in bytes.",
+		},
+	)
+
+	m.reqSz = prometheus.NewSummary(
+		prometheus.SummaryOpts{
+			Subsystem: "pocket_id",
+			Name:      "request_size_bytes",
+			Help:      "The HTTP request sizes in bytes.",
+		},
+	)
+
+	prometheus.Register(m.reqCnt)
+	prometheus.Register(m.reqDur)
+	prometheus.Register(m.resSz)
+	prometheus.Register(m.reqSz)
+
+	return m
+}
+
+func (m *PrometheusMetricsMiddleware) Add() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		// Skip /metrics URL calls, as promhttp already tracks them
+		if c.Request.URL.Path == "/metrics" {
+			c.Next()
+			return
+		}
+
+		start := time.Now()
+		reqSz := computeApproximateRequestSize(c.Request)
+
+		c.Next()
+
+		status := strconv.Itoa(c.Writer.Status())
+		elapsed := float64(time.Since(start)) / float64(time.Second)
+		resSz := float64(c.Writer.Size())
+
+		m.reqDur.WithLabelValues(status, c.Request.Method, c.Request.URL.Path).Observe(elapsed)
+		m.reqCnt.WithLabelValues(status, c.Request.Method, c.HandlerName(), c.Request.Host, c.Request.URL.Path).Inc()
+		m.reqSz.Observe(float64(reqSz))
+		m.resSz.Observe(resSz)
+	}
+}