fix golint issues in zrpc (#531)

zrpc/internal/serverinterceptors/tracinginterceptor.go

@@ -8,6 +8,7 @@ import (
 	"google.golang.org/grpc/metadata"
 )
 
+// UnaryTracingInterceptor returns a func that handles tracing with given service name.
 func UnaryTracingInterceptor(serviceName string) grpc.UnaryServerInterceptor {
 	return func(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo,
 		handler grpc.UnaryHandler) (resp interface{}, err error) {