Implement process uptime metric (#666)

* Implement process uptime metric
* Add clarification comment
* Add another clarification comment

pkg/metrics/handler.go

@@ -0,0 +1,25 @@
+package metrics
+
+import (
+	"net/http"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+type Handler struct {
+	startTime   time.Time
+	promHandler http.Handler
+}
+
+func (h Handler) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
+	uptimeGauge.Set(time.Since(h.startTime).Seconds())
+	h.promHandler.ServeHTTP(rw, req)
+}
+
+func NewHandler() http.Handler {
+	return Handler{
+		startTime:   time.Now(),
+		promHandler: promhttp.Handler(),
+	}
+}