gistfile1.txt

Wow, everytime I think I'm done.  I find another hurdle.

Client and server are identical for my uses.  So both client and server 
should track the public key of their peers.  So Server/Client handshake 
can track who I'm talking to, last time I saw them, IP address, and public key.

Now when an RPC call is made I want to use UnaryServerInterceptor and 
StreamServerInterceptor to allow every call to check if the Peer has 
been tracked, if not look up the public key and set the PeerID.
        peerID, ok := ctx.Value(peerIDKey).(int)
        if (ok == false ) {
                  pubKey=GetPubKey(...)
                  peerId=getClientOrAdd((pubKey)
        }
        <answer RPC knowing which peer I'm talking to>

I nice example of using context for this kind of metadata is:
    https://blog.golang.org/context

In particular:
   https://blog.golang.org/context/userip/userip.go

Which has:
func NewContext(ctx context.Context, userIP net.IP) context.Context {
   return context.WithValue(ctx, userIPKey, userIP)
}

Problem is, my RPC calls trigger a ctx being passed to the Interceptor, but 
then I can't change the ctx.  So how do I make updates to the ctx?

My attempt is at https://github.com/spikebike/grpc-go/blob/ssh-style-auth/examples/route_guide/server/server.go,
but complains:
go run server/server.go
server/server.go:246: cannot assign to ctx.Value(0).(int)