renamed stats.Handler to grpc.StatsHandler

To avoid confusion about stats and be more explicit about the type our gRPC middleware implements I have renamed stats.Handler to grpc.StatsHandler

Author: basvanbeek

README.md

@@ -55,7 +55,7 @@ For convenience `NewClient` is provided which returns a HTTP Client which embeds
 calling the `DoWithAppSpan()` method.
 
 #### grpc
-Easy to use stats.Handler middleware are provided for tracing gRPC server and
+Easy to use grpc.StatsHandler middleware are provided for tracing gRPC server and
 client requests. 
 
 For a server, pass `NewServerHandler` when calling `NewServer`, e.g.,