close gRPCReporter error

[lokichoggio]

when close gRPCReporter, error like this

go2sky-gRPC2021/09/22 17:03:57 send closing error rpc error: code = Canceled desc = grpc: the client connection is closing go2sky-gRPC2021/09/22 17:03:57 rpc error: code = Canceled desc = grpc: the client connection is closing

if r.sendCh != nil，r.closeStream and r.closeGRPCConn (line 268) maybe called after r.closeGRPCConn (line 236)

https://github.com/SkyAPM/go2sky/blob/b16c9f07fef6a007b94f940566af4374a5bf44dc/reporter/grpc.go#L232-L237

https://github.com/SkyAPM/go2sky/blob/b16c9f07fef6a007b94f940566af4374a5bf44dc/reporter/grpc.go#L247-L273

[lokichoggio]

add else

func (r *gRPCReporter) Close() {
    if r.sendCh != nil {
        close(r.sendCh)
    } else {
        r.closeGRPCConn()
    }
}

[wu-sheng]

You could send a pull request if you like. @arugal could review from there.