Adds request latency tracking

Fixes #1

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
+}