disable prometheus if not configured (#663)

zrpc/internal/serverinterceptors/prometheusinterceptor.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/tal-tech/go-zero/core/metric"
+	"github.com/tal-tech/go-zero/core/prometheus"
 	"github.com/tal-tech/go-zero/core/timex"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/status"
@@ -36,6 +37,10 @@ var (
 func UnaryPrometheusInterceptor() grpc.UnaryServerInterceptor {
 	return func(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (
 		interface{}, error) {
+		if !prometheus.Enabled() {
+			return handler(ctx, req)
+		}
+
 		startTime := timex.Now()
 		resp, err := handler(ctx, req)
 		metricServerReqDur.Observe(int64(timex.Since(startTime)/time.Millisecond), info.FullMethod)