Adds request latency tracking

Fixes #1

cmd/hwtd/main.go

@@ -6,7 +6,9 @@ import (
 	"os"
 
 	"github.com/prometheus/client_golang/prometheus/promhttp"
+
 	"mcquay.me/hwt"
+	"mcquay.me/hwt/metrics"
 	pb "mcquay.me/hwt/rpc/hwt"
 )
 
@@ -15,8 +17,14 @@ func main() {
 	if err != nil {
 		log.Fatalf("cannot get hostname: %v", err)
 	}
+
+	if err := metrics.RegisterPromMetrics(); err != nil {
+		log.Fatalf("registering prom metrics: %v", err)
+	}
+
 	s := &hwt.Server{hn}
-	th := pb.NewHelloWorldServer(s, nil)
+	hs := hwt.NewMetricsHooks(metrics.HTTPLatency)
+	th := pb.NewHelloWorldServer(s, hs)
 	sm := http.NewServeMux()
 	sm.Handle("/", th)
 	sm.Handle("/metrics", promhttp.Handler())

hooks.go

@@ -0,0 +1,46 @@
+package hwt
+
+import (
+	"context"
+	"time"
+
+	"github.com/twitchtv/twirp"
+)
+
+var reqStartKey = new(int)
+
+type Timer func(path string, dur time.Duration)
+
+func NewMetricsHooks(timer Timer) *twirp.ServerHooks {
+	hs := &twirp.ServerHooks{}
+
+	hs.RequestReceived = func(ctx context.Context) (context.Context, error) {
+		return markReqStart(ctx), nil
+	}
+
+	hs.ResponseSent = func(ctx context.Context) {
+		name, ok := twirp.MethodName(ctx)
+		if !ok {
+			// XXX (sm) : something else?
+			panic("missing name")
+		}
+		start, ok := getReqStart(ctx)
+		if !ok {
+			// XXX (sm) : something else?
+			panic("missing start")
+		}
+		dur := time.Now().Sub(start)
+		timer(name, dur)
+	}
+
+	return hs
+}
+
+func markReqStart(ctx context.Context) context.Context {
+	return context.WithValue(ctx, reqStartKey, time.Now())
+}
+
+func getReqStart(ctx context.Context) (time.Time, bool) {
+	t, ok := ctx.Value(reqStartKey).(time.Time)
+	return t, ok
+}

metrics/metrics.go

@@ -0,0 +1,9 @@
+package metrics
+
+import (
+	"time"
+)
+
+func HTTPLatency(path string, dur time.Duration) {
+	httpReqLat.WithLabelValues(path).Observe(float64(dur) / float64(time.Millisecond))
+}

metrics/prom.go

@@ -0,0 +1,23 @@
+package metrics
+
+import (
+	"github.com/pkg/errors"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+var (
+	httpReqLat = prometheus.NewSummaryVec(
+		prometheus.SummaryOpts{
+			Name: "hwt_request_latency_ms",
+			Help: "Latency in ms of http requests grouped by req path",
+		},
+		[]string{"path"},
+	)
+)
+
+func RegisterPromMetrics() error {
+	if err := prometheus.Register(httpReqLat); err != nil {
+		return errors.Wrap(err, "registering http request latency")
+	}
+	return nil
+}