Handle gatherer correctly

We need to recreate the handler for the gatherers every time we add a
new one.

options.go

@@ -90,7 +90,7 @@ func WithMetricsHandler() Option {
 		s.Router.Handle("/metrics",
 			promhttp.InstrumentMetricHandler(
 				s.internalRegister, /* Register */
-				promhttp.HandlerFor(s.gatherers, promhttp.HandlerOpts{})))
+				http.HandlerFunc(s.metricsHandler)))
 
 		return nil
 	}

svc.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"go.uber.org/zap"
 )
 
@@ -43,6 +44,7 @@ type SVC struct {
 
 	gatherers        prometheus.Gatherers
 	internalRegister *prometheus.Registry
+	promHander       http.Handler
 }
 
 // New instantiates a new service by parsing configuration and initializing a
@@ -100,6 +102,7 @@ func (s *SVC) AddWorker(name string, w Worker) {
 }
 
 func (s *SVC) AddGatherer(gatherer prometheus.Gatherer) {
+	s.promHander = nil
 	s.gatherers = append(s.gatherers, gatherer)
 }
 
@@ -231,3 +234,10 @@ func (s *SVC) recoverWait(name string, wg *sync.WaitGroup, errors chan<- error)
 		}
 	}
 }
+
+func (s *SVC) metricsHandler(w http.ResponseWriter, r *http.Request) {
+	if s.promHander == nil {
+		s.promHander = promhttp.HandlerFor(s.gatherers, promhttp.HandlerOpts{})
+	}
+	s.promHander.ServeHTTP(w, r)
+}