track http status codes in metrics

cmd/hwtd/main.go

@@ -23,7 +23,7 @@ func main() {
 	}
 
 	s := &hwt.Server{hn}
-	hs := hwt.NewMetricsHooks(metrics.HTTPLatency)
+	hs := hwt.NewMetricsHooks(metrics.HTTPLatency, metrics.HTTPCode)
 	th := pb.NewHelloWorldServer(s, hs)
 	sm := http.NewServeMux()
 	sm.HandleFunc("/", hwt.Auth(th.ServeHTTP))

hooks.go

@@ -10,8 +10,9 @@ import (
 var reqStartKey = new(int)
 
 type Timer func(path string, dur time.Duration)
+type Statuser func(path, status string)
 
-func NewMetricsHooks(timer Timer) *twirp.ServerHooks {
+func NewMetricsHooks(timer Timer, status Statuser) *twirp.ServerHooks {
 	hs := &twirp.ServerHooks{}
 
 	hs.RequestReceived = func(ctx context.Context) (context.Context, error) {
@@ -31,6 +32,12 @@ func NewMetricsHooks(timer Timer) *twirp.ServerHooks {
 		}
 		dur := time.Now().Sub(start)
 		timer(name, dur)
+
+		sc, ok := twirp.StatusCode(ctx)
+		if !ok {
+			panic("missing code")
+		}
+		status(name, sc)
 	}
 
 	return hs

metrics/metrics.go

@@ -7,3 +7,7 @@ import (
 func HTTPLatency(path string, dur time.Duration) {
 	httpReqLat.WithLabelValues(path).Observe(float64(dur) / float64(time.Millisecond))
 }
+
+func HTTPCode(path string, status string) {
+	httpStatus.WithLabelValues(path, status).Inc()
+}

metrics/prom.go

@@ -21,11 +21,22 @@ var (
 		},
 		[]string{"path"},
 	)
+
+	httpStatus = prometheus.NewCounterVec(
+		prometheus.CounterOpts{
+			Name: "hwt_http_requests_total",
+			Help: "How many HTTP requests processed, partitioned by status code and HTTP method.",
+		},
+		[]string{"path", "code"},
+	)
 )
 
 func RegisterPromMetrics() error {
 	if err := prometheus.Register(httpReqLat); err != nil {
 		return errors.Wrap(err, "registering http request latency")
 	}
+	if err := prometheus.Register(httpStatus); err != nil {
+		return errors.Wrap(err, "registering http request status")
+	}
 	return nil
 }