Add /p2p page (#3391)

* add /p2p page

* fix tests

shared/prometheus/service.go

@@ -24,9 +24,15 @@ type Service struct {
 	failStatus  error
 }
 
+// Handler represents a path and handler func to serve on the same port as /metrics, /healthz, /goroutinez, etc.
+type Handler struct {
+	Path string
+	Handler func(http.ResponseWriter, *http.Request)
+}
+
 // NewPrometheusService sets up a new instance for a given address host:port.
 // An empty host will match with any IP so an address like ":2121" is perfectly acceptable.
-func NewPrometheusService(addr string, svcRegistry *shared.ServiceRegistry) *Service {
+func NewPrometheusService(addr string, svcRegistry *shared.ServiceRegistry, additionalHandlers ...Handler) *Service {
 	s := &Service{svcRegistry: svcRegistry}
 
 	mux := http.NewServeMux()
@@ -34,6 +40,11 @@ func NewPrometheusService(addr string, svcRegistry *shared.ServiceRegistry) *Ser
 	mux.HandleFunc("/healthz", s.healthzHandler)
 	mux.HandleFunc("/goroutinez", s.goroutinezHandler)
 
+	// Register additional handlers.
+	for _, h := range additionalHandlers {
+		mux.HandleFunc(h.Path, h.Handler)
+	}
+
 	s.server = &http.Server{Addr: addr, Handler: mux}
 
 	return s