create a gin middleware to collect request metrics and export them to the Prometheus route

metric/request.go

@@ -0,0 +1,78 @@
+package metric
+
+import (
+	"strconv"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+const (
+	favicon = "/favicon.ico"
+)
+
+// Prometheus contains the metrics gathered by the instance and its path
+type Prometheus struct {
+	reqCnt *prometheus.CounterVec
+	reqDur *prometheus.HistogramVec
+}
+
+// NewPrometheus generates a new set of metrics with a certain subsystem name
+func NewPrometheus() (*Prometheus, error) {
+	reqCnt := prometheus.NewCounterVec(
+		prometheus.CounterOpts{
+			Namespace: namespaceAPI,
+			Name:      "requests_total",
+			Help:      "How many HTTP requests processed, partitioned by status code and HTTP method",
+		},
+		[]string{"code", "method", "path"},
+	)
+	if err := registerCollector(reqCnt); err != nil {
+		return nil, err
+	}
+	reqDur := prometheus.NewHistogramVec(
+		prometheus.HistogramOpts{
+			Namespace: namespaceAPI,
+			Name:      "request_duration_seconds",
+			Help:      "The HTTP request latencies in seconds",
+		},
+		[]string{"code", "method", "path"},
+	)
+	if err := registerCollector(reqDur); err != nil {
+		return nil, err
+	}
+	return &Prometheus{
+		reqCnt: reqCnt,
+		reqDur: reqDur,
+	}, nil
+}
+
+// PrometheusMiddleware creates the prometheus collector and
+// defines status handler function for the middleware
+func PrometheusMiddleware() (gin.HandlerFunc, error) {
+	p, err := NewPrometheus()
+	if err != nil {
+		return nil, err
+	}
+	return p.Middleware(), nil
+}
+
+// Middleware defines status handler function for middleware
+func (p *Prometheus) Middleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		if c.Request.URL.Path == favicon {
+			c.Next()
+			return
+		}
+		start := time.Now()
+		c.Next()
+
+		status := strconv.Itoa(c.Writer.Status())
+		elapsed := float64(time.Since(start)) / float64(time.Second)
+		fullPath := c.FullPath()
+
+		p.reqDur.WithLabelValues(status, c.Request.Method, fullPath).Observe(elapsed)
+		p.reqCnt.WithLabelValues(status, c.Request.Method, fullPath).Inc()
+	}
+}