fix(metrics): enable prometheus global switch when user enable devsever metrics (#3169)

internal/devserver/server.go

@@ -10,6 +10,7 @@ import (
 	"github.com/felixge/fgprof"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/zeromicro/go-zero/core/logx"
+	"github.com/zeromicro/go-zero/core/prometheus"
 	"github.com/zeromicro/go-zero/core/threading"
 	"github.com/zeromicro/go-zero/internal/health"
 )
@@ -42,6 +43,8 @@ func (s *Server) addRoutes() {
 
 	// metrics
 	if s.config.EnableMetrics {
+		// enable prometheus global switch
+		prometheus.Enable()
 		s.handleFunc(s.config.MetricsPath, promhttp.Handler().ServeHTTP)
 	}
 	// pprof