Grpc Reporter's logger cannot use custom logger.

[legenove]

The gRPCReporter struct's logger is *log.Logger. User must use *log.Logger. Bug some logger middleware can‘t convert to *log.Logger. It is recommended to use an interface here, like:

type Logger interface {
    Printf(format string, v ...interface{})
}

type gRPCReporter struct {
    ...
    logger           Logger
    ...
}