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())