disable prometheus if not configured (#663)

zrpc/internal/clientinterceptors/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"
@@ -35,6 +36,10 @@ var (
 // PrometheusInterceptor is an interceptor that reports to prometheus server.
 func PrometheusInterceptor(ctx context.Context, method string, req, reply interface{},
 	cc *grpc.ClientConn, invoker grpc.UnaryInvoker, opts ...grpc.CallOption) error {
+	if !prometheus.Enabled() {
+		return invoker(ctx, method, req, reply, cc, opts...)
+	}
+
 	startTime := timex.Now()
 	err := invoker(ctx, method, req, reply, cc, opts...)
 	metricClientReqDur.Observe(int64(timex.Since(startTime)/time.Millisecond), method)