Implement process uptime metric (#666)

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

pkg/metrics/server.go

@@ -3,17 +3,12 @@ package metrics
 import (
 	"net/http"
 
-	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/sirupsen/logrus"
 )
 
-func Handler() http.Handler {
-	return promhttp.Handler()
-}
-
 func StartServer(logger *logrus.Logger, path string, addr string) error {
 	logger.WithField("addr", addr).WithField("path", path).Info("starting prometheus metrics server")
 
-	http.Handle(path, Handler())
+	http.Handle(path, NewHandler())
 	return http.ListenAndServe(addr, nil)
 }