fix build on net4.0
fix build on net4.0

MetaG8/Metrics.NET,mnadel/Metrics.NET,etishor/Metrics.NET,DeonHeyns/Metrics.NET,alhardy/Metrics.NET,huoxudong125/Metrics.NET,ntent-ad/Metrics.NET,DeonHeyns/Metrics.NET,mnadel/Metrics.NET,Recognos/Metrics.NET,etishor/Metrics.NET,huoxudong125/Metrics.NET,MetaG8/Metrics.NET,Liwoj/Metrics.NET,cvent/Metrics.NET,Recognos/Metrics.NET,ntent-ad/Metrics.NET,cvent/Metrics.NET,alhardy/Metrics.NET,Liwoj/Metrics.NET,MetaG8/Metrics.NET